Does using a dice shard on a book in the book store bring another book or random item?

[DuelShockX]

As title says

[Nixmachine]

If you picked all the books - random item
If you didn't pick up all the books in the library item pool - another book

[benjineel]

It gives a book, unless you have managed to empty the Library Item pool by having every item in the Library pool spawned once, in which case it will spawn a random item from the Treasure Room Item pool (and if the Treasure Room Itema pool is somehow depleated you will start spawning Breakfast). The Library Item pool is one of the smallest Item pools (12 items even with all the DLC ) in the game, and some of the Library rooms can have a lot of pedastals (3-5) so it can be a common strategy, especially if you have some means of more reliablly rerolling ( the D6, a rune bag to dig for Perthro, or blank card + Perthro/Dice shard etc.) to reroll Libraries until the pool depletes (plus if you touch 3 books with afterbirth + you get the Bookworm transformation) turning Libraries into potenially better Treasure rooms.

[Nazemec]

Originally posted by benjineel:

It gives a book, unless you have managed to empty the Library Item pool by having every item in the Library pool spawned once

Taken* once. Seeing an item isn't always enough to remove it from its current pool, depending on the pool. I tested it a while ago.
Unless it got changed with AB+, but I highly doubt a core mechanic would be changed that much.