A NPC in my adventure (set in Eberron), which I lead as DM, wants to replace some maps in a book with different maps. The person removes the maps by cutting out the pages containing them and somehow inserts the different maps to "repair" the missing pages. How can they do this?
The motivation of the person is to mislead the party without leaving a trace.
Mending does not work, according to this Q&A: Can the Mending cantrip affix any surface to any other surface?
If your NPC cuts the page from the book and joins it this might be quite obvious. However, if they pull the book apart, remove the entire signature (which will usually be several pages - in most modern books an octavo [16] pages but older books were often folios [4 pages]), replace that and rebind the book.
This isn't hard or particularly time-consuming if you know how and have the equipment and would be virtually undetectable. The only clues would be the quality of the paper and the forgery.
As the DM, you are not limited by the mechanics of the game. You don't have to have a mechanical justification for every story element. If one of the challenges you want to present to the party is an atlas with forged, misleading maps in it, just do that. It's your world, and you are in charge of it. The rules are not in charge:
The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.
-Introduction, Dungeon Master's Guide
It shouldn't matter if you don't give a neat explanation based on the rules available to players. You don't do this with other story elements, and you shouldn't have to do it here. Was that tavern built according to RAW? No, of course not, it's a story element, nobody built it, I put it there because it's part of the story. Same with your book here. It is entirely feasible to do what you describe without using any spell or magic item, assuming you have the right skills. So that's what happened.
Practically IRL this is very difficult. Books are made out of signatures, which are multi-page sheets folded to make the individual pages. Perfect binding, where pages are glued to the spine, like a paperback, is a very recent invention.
Since this is part of your world-building, you can have it done however you want. But I would give the player characters the chance to make a check to uncover the scam. Depending on how skilled your imaginary forger is, a DC of between 15 and 20 for and Intelligence Investigation check makes sense to me. If it is key to the plot, make it undetectable via any means.
The fabricate spell can turn raw materials into a crafted object. The book could be considered to require a level of craftsmanship, as mentioned by the spell, so the NPC needs proficiency with the forgery kit. Ideally you could have the NPC make a modified duplicate. You might be able to use the original book as materials for the fabricate spell although that might be stretching or breaking the rules. As DM it is your decision though.
You already stated that Mending can't affix an arbitrary surface to another. However in this case that's not what you need to do.
You have a book which you have torn a page out of. You have a new page. Mending "repairs a single break or tear in an object you touch" and that's exactly what you need to do, repair a single tear in a book.
Some may argue that you have to have the original parts, but that isn't strictly the case. If you call a plumber to repair your toilet, an electrician to repair a blown bulb, a mechanic to repair your blown head gasket, or a plasterer to repair a chipped wall, all of these people are going to use new material which was not present in the original in their repairs.
It's completely reasonable for a book with a page torn out to be repaired using a new page.
For those wondering if an object repaired with material that was not present in the original, I recommend you check out the wiki article on the Ship of Theseus. In everyday language I think this is a solved problem; if someone tells you their car had a flat and they had to get a new tire, you wouldn't say they got a new car!
A bit of a frame challenge here, but don't rebind or fix the book. Just destroy it.
Cut out all the pages, take the ones you want to conceal. And the cover/spine. Replace the pages (making sure to rough up the edges). Maybe add some additional artful distressing to both the real and fakes pages, to make the replacement harder to spot. Or even replace smore of the original pages (the ones you intended to leave) with exact duplicates, to help the fakes fit.
You could also take some of the unimportant maps, to make it look like that is why the book was damaged.
Then stuff them in a folder, or tie them up with twine. Or slip them all back into cover, if you don't want to take it, or the cover is important (and then you could still tie it with twine).
The real key here is making sure the PCs don't have any magic that can detect the forgery or reason for the damage (i.e. psychometry). Fortunately, those are fewer and far between in 5E.
Rather than removing and replacing the maps in the original book, he could bleach the pages to remove all the ink, and then draw new maps on the now-blank pages. This would of course require the nefarious NPC to have access to the book for long enough for the bleach to do its work, and then for it to evaporate and for the new ink to dry (otherwise, a keen observer might detect the smell of bleach and/or fresh ink). Alternatively, if the replacement maps are close enough to the originals, with just some key details altered for the purposes of the deception, he could simply bleach out small parts of the page and redraw only those with his changes.
Since this is an Eberron question, let me give an Eberron answer. Consider that a valuable sort of book might actually be a Spellshard (E:RFtLW pg 279). This allows for the information to be protected by a password, and is generally less fragile than a normal book. And in Eberron, common magic items like this are, well, commonplace. Your villain needs to have access to the password, but at that point forgery may not be as easily detectable.
