Magic bookshelf dungeon feature, which is heavily inspired by and is functionally similar to the TNNT swap chest, but only involves spellbooks instead of all items (and is not itself an object).

Looting while standing on it gives you a feeling you should put a book on the empty shelf, and if you do, you magically find other books on the shelf. You can see the identities of these books but do not permanently learn them if you didn’t previously know them. Once you take a book from the shelf, it vanishes.

Left up to the implementor whether the books would actually come from other players like the TNNT swap chest does (in which case it might end up filled with low-value or common books; if so, there are various ways this could be mitigated such as the shelf not accepting a book type it already has, or adding in randomly generated books) or whether the books would just be a pool of random ones selected at the start of the game.

An additional possible detail is that you can only see books on the shelf of equal or lower level than the total level of books you yourself have put in. So if you put in a level 1 book, you will only see other level 1 books, but if you then add a level 3 book, you will see books of up to level 4. Or, you always see all books, but only books below this threshold show you the spell and others appear as “a spellbook”. identities