after messing around with the chiseled bookshelf I realized that when you take the books out, it doesn't update the comparator connected to the bookshelf, and I had to replace it every time

steps:

1. place chiseled bookshelf down
2. place comparator connecting to bookshelf on normal or subtract mode
3. place 9 redstone dust extending from the comparator
4. put any number of any type of book into the chiseled bookshelf in any slot
5. take the books out of the chiseled bookshelf

what you would expect:

the redstone and comparator would turn off once all books were removed (like other blocks such as hoppers, chests, barrels, furnaces, blast furnaces, smokers, ETC)

what happens:

redstone stays on until chiseled bookshelf is broken/replaced