Can a multiclass Archivist-Wizard benefit from class features that affect the spellbook class feature?
Class "Archivist" from Heroes of Horror book is a divine class that works mostly like a wizard only with divine spells instead of arcane.
In order to keep a list of available spells, this class uses a Prayerbook, which is described as a Wizard's spellbook, only for divine spells:
Prayerbook: Unlike a cleric, an archivist does not receive his daily spell complement from whatever deity or cosmic force he worships. Rather, he must seek out and collect new spells much as a wizard does, but from such esoteric sources as holy tablets, ancient steles, or other magical scriptures. He cannot prepare any spell not recorded in his prayerbook except for read magic, which archivists can prepare from memory.
An archivist begins play with a prayerbook containing all 0-level cleric spells plus three 1st-level cleric spells of the player's choice. For each point of Intelligence bonus the archivist has, the prayerbook has an additional 1st-level cleric spell. At each new class level, the archivist gains two new cleric spells for his prayerbook; these can be of any spell level or levels that he can cast (based on hisnew archivist level). At any time, an archivist can also add spells found on scrolls containing divine spells to his prayerbook, but he must make any rolls and spend the time required (see Adding Spells to a Wizard's Spellbook on page 178 of the Player's Handbook). The archivist can learn and thus prepare nonclerical divine spells in this fashion but the two free spells he gains for advancing in class level must be selected from the cleric spell list.
Since it is like a spellbook, some stuff that improves spellbook usage should affect the prayerbook as well. Some examples would be:
- Geometer prestige class in Complete Arcane can reduce the number of pages used in a spellbook when writing a spell
- Eidetic Spellcaster alternative class feature allows a Wizard to keep a spellbook in memory instead of in physical form (but still need to spend money per page)
- Spell Mastery feat allows a spellcaster to cast certain spells as if that spell was in the caster's spellbook
- Boccob's Blessed Book is a wondrous item that permits writing spells in it without spending so much money
- Etc.
In my opinion this would follow Rules As Intended (i.e. it functions like a spellbook) and it would not be too overpowered (you are losing levels of one class to get levels from the other one).