Problem:

It gives power with the number of books inside but it will start removing power levels at the second time removing a book.

If there are 4 books and you remove one, the power stays on level 4, if you remove another one one more it goes to level 3.

Therefore after one book was put the power of the block will stay at least 1 forever because there can't be -1 books in the bookshelf.

Place a chiseled bookshelf and a redstone comparator behind it, then place 6 redstone in a row and a redstone lamp next to each. Put books and remove them to test the bug.

It's a small bug but very bad for redstoners.

Thank You.

(Sorry for my bad english)