What is the purpose of the wizard's spellbook, from a mechanical standpoint?

Question

I'm still fairly new to DMing, and the wizard's spellbook is constantly breaking my mind. While I think I finally understand how it is working, I wonder why it is in the game to begin with. And I don't mean from an in-game/lore/flavor standpoint, but from a mechanical standpoint.

As far as I can see, the spellbook only makes things more difficult for the wizard. While all other spellcasting classes get access to the entire list of spells they have slots for, the wizard only has access to those in his book (a number of spells equal to 6 + (2 * level ups) + whatever they find in other spellbooks).
Is there anything the wizard gets in return for that which other classes can't do?

Answer 1

I have two observations, which don’t strictly have anything to do with the spellbook per se, but rather about the context in which the wizard exists, and then I’ll note a number of advantages and disadvantages that the wizard has due to the spellbook.

Observation: the wizard is a “prepared caster”

The wizard casts spells by preparing a subset of those they have available each morning. This is different from some other spellcasters (sometimes called “spontaneous”), that simply know a fixed selection of spells and just casts those without preparation. Other prepared casters include the cleric and druid, and contrast with classes like bard or sorcerer.

Preparation is generally an advantage: you can choose spells relevant for that particular day, instead of having to choose spells that will be relevant for the rest of your life. Spontaneous spellcasters do get somewhat more spells known than a prepared caster can prepare on any given day, particularly at high levels, which mitigates this advantage somewhat, but in my experience preparation is still decidedly the better way to go. (In a vacuum; obviously every spellcasting class also has other potent features that mean you are never just picking “prepared” vs. “spontaneous.” This kind of “theoretical, in a vacuum comparison” vs. “contextual, in practice comparison” is a big theme with this anwer.)

Either way, though, it makes the most sense to compare the wizard’s spellcasting to the cleric and druid, since they’re the most comparable classes.

Observation: the wizard spell list is massive

The wizard’s spell list is simply a great deal larger than other classes’. That has a lot of influence over how we should look at how the wizard’s spellcasting works.

Disadvantage: no immediate access to the entire wizard spell list

Compared to other prepared casters—the aforementioned cleric and druid—the wizard has to jump through extra hoops to prepare their spells. Clerics and druids just get to prepare any and all cleric and druid spells, respectively, as soon as they reach the appropriate level.

The wizard cannot. The wizard has to get them into their spellbook first. They get a few for free as they level up, but the rest require time and money. That’s a drawback.

Note, however, that this directly goes to the advantage we observed previously: the wizard spell list is massive. That means that, while at first the wizard may not have immediate access to as many spells as a cleric or druid, if they are diligent about filling up their spellbook they can eventually gain access to many, many more spells.

If one had access to the wizard spell list, but didn’t have to worry about a spellbook, that would obviously be better. But that isn’t an option that the game is presenting. Instead, you have a trade-off: deal with a spellbook, and theoretically get access to a lot more spells. It’s an attempt at balance.

Note that, compared to spontaneous classes, the wizard’s access is still superior: a wizard gets to add more spells to their spellbook for free, automatically, than a spontaneous class can learn in a given level.

Disadvantage: major weak point

A wizard is dependent on their spellbook; that makes it a major vulnerability.

How large a disadvantage this is in practice varies widely, though: a spellbook is so core and iconic to a wizard that many DMs avoid taking it away as “un-fun.” A wizard’s complete spellbook is so valuable, and so physically massive, that past editions had rules and lore around keeping backup copies and using smaller traveling spellbooks with fewer spells, as well as fancy magic book covers and the like to protect them. The ideal case, perhaps, is that the wizard puts some effort into these sorts of options in order to protect the spellbook, but those protections are sufficient, so rather than being a binary issue there is a lesser cost associated. Landing on that sweet spot requires a fair amount of same-page-ness between player and DM, but then, so does most of D&D. Note, however, that changes to spell preparation in 5e make a lost spellbook not nearly as crippling for a wizard as it had been in pre-4e editions (the 4e wizard was completely different and not at all comparable to other editions’).

Advantage: Greater ritual access

Unlike clerics and druids, who must prepare a spell in order to cast it as a ritual, a wizard just has to have it in their spellbook. That allows them to save their prepared spells for other things, considerably amplifying the advantages of ritual casting.

Advantage, sorta: Arcane Recovery

The wizard’s Arcane Recovery feature is potent, and requires a spellbook. Again, in the theoretical case of Arcane Recovery being available without a spellbook, that would be better—but again you don’t have that option. It’s kind of hard to really say that this is an advantage to having a spellbook, per se, but it’s definitely part of the overall advantage of being a wizard.

Answer 2

The Wizard gains all the benefits of being a prepared caster, has an extremely large spell list to choose from, and gets a few other benefits from the class

Because the Wizard is a prepared caster, they can change out their spell list with each long rest. This adds a great amount of versatility that non-prepared casters (Bard, Sorcerer, Warlock, etc...) do not have. Thus, the Wizard has an advantage over non-prepared casters.

The Wizard also has an advantage over the other prepared casters because their spell list is huge. The Wizard gets a whopping 308 spells while the Cleric gets only 123; the Wizard gets 2.5 times as many spells as Clerics and this gives them a benefit over them. Sure, the Wizard cannot prepare any of those spells each day; but they can curate the list that they can prepare spells from from this much larger list, allowing for a great deal of customization and versatility.

Another benefit Wizards have over some casters is that they can cast spells as rituals, including unprepared ones that are just sitting in their spellbook.

Another benefit almost unique to the Wizard is their Arcane Recovery feature which lets them restore spell slots after a short rest one per day.

Another interesting fact of the Wizard is that they can take a scroll for a spell and put it into their spellbook, thus increasing the pool from which they can prepare spells. The fact that they have to do this in the first place could be considered a disadvantage but the fact that they can do it means the Wizard has a way to theoretically put all of their spells into their spellbook. Thus, a Wizard could have 2.5 times as many spells as a Cleric to choose from each day which is a significant advantage.

Answer 3

They get a few things

1. Wizards are able to cast ritual spells that are scribed into their spellbook, even if it they are not spells they have prepared.
2. Wizards are able to recover a certain number of spell slots during a short rest by studying their spellbook. This is almost unique, there is a druid archetype that can do the same.
3. Unlike other non-divine casters, wizards can attempt to acquire new spells from scrolls and spellbooks they obtain without having to gain a level first, as long as the spell is on their spell list.

Keep in mind, wizards and spellbooks have been part of D&D lore since the beginning. If you took away their spellbook, how would the class even function?

Answer 4

I think what you're missing is that most arcane casters are limited by a "spells known" attribute. They don't have full access to their class's full spell list and don't prepare spells in the morning; instead, when they level up, they get to choose a very small number of spells off their class spell list to add to their known spells, and those are the only spells they can ever cast. Generally, they can swap out one known spell each time they level, but there's very little flexibility to it. Whatever you picked, you're largely stuck with in the long term.

Wizards are less limited. At any given level, a wizard always has roughly twice as many spells in their spellbook as any know-spell caster's known list, and can have even more if they spend the money and effort to write in additional spells from external sources. They can't have all those available to cast at once, thus the preparation limits, but they're far more flexible than other arcane casters (though not as flexible as the divine casters).

Rituals only enhance that flexibility. Most casters must either know or prepare a spell to be able to cast it as a ritual -- but not Wizards. If it's in the book, they can do the ritual for it, no need to spend a precious daily preparation slot on it first.

Answer 5

I think @KRyan's answer does a great job of discussing the mechanical benefits and drawbacks of the wizard's spellbook.

One thing I haven't seen mentioned, though, is that the spellbook gives wizards a unique hook: They get a permanent*, mechanical boost from finding magical knowledge. While all characters can get stronger by acquiring experience and magical items, wizards are special since they can also improve simply by finding new spells around the world. For the wizard player, sneaking around a library of forbidden secrets isn't just about the plot or the gold, they might get to add a few new spells to their character sheet, too.

*permanent assuming the spellbook isn't lost or destroyed.