In a recent snapshot, a bug was fixed where knowledge books in a stack of more than 1 would consume the entire stack at once upon use. It seems that by fixing that bug, something else broke in the case of a stack-size of 1: they just no longer work at all.

Here's an example of the observed behavior:

1. Type /give @s knowledge_book[recipes=["minecraft:flint_and_steel"]]
2. Making sure you don't have the flint_and_steel recipe unlocked already, go to survival mode and use the knowledge book. The recipe won't be unlocked.
3. Now give yourself another one, go to Creative mode, and use it. It WILL unlock the recipe.
4. Type /give @s knowledge_book[recipes=["minecraft:flint_and_steel"],max_stack_size=2] 2
5. Revoke the recipe, go back to survival, and use the book. This time the recipe will be unlocked.

It seems like there's an issue with the case where the stack no longer exists after using the book (ie, the stack only has a size of 1, and the book is consumed, so you must not be in creative mode).

Thanks to roseprose for originally pointing this out!