Hello!! well for the answer about ıf the books only exist for content: yes and no
here is some spoilers:
While books in library usually exists to let you know the world's lore and what happened in past (MC' past, worlds general situation, knowledge about Ashborn duchy etc) but there is also a book that is crutial to open a special scene with İsaac. Plus with that scene happening a few future events will also have small changes due to certain choices (ofc ıf you choose those certain actions)
Just in case you want to see the scene:
After ahving a dinner event with him you can go to library and read the ''Ritual of Sacred Ashes'' after reading this one you can get out of the library for next needed steps:
you should go to west wing, where isaac is and get into his room after having a cht or asking him to ''give you something to eat'' he will you bottle certain liquid (edit: ı forgot to add a small detail, ıf you ask him for something to eat the scene will automatically go to next step but ıf you chat with him first then nathainel comes and tells you taht ıts İsaac's eating tie and they can either drink with him or go away, to reach the special scene you first need to act drinking with so you can see the drink's content...well he is a vampire so you can guess what is it). when drink is poured in front of you refuse to drink and tell him that ıts not morally right to do such thing. well while ı wont say much thing for hidden scene but lets say do not have mercy on him. he asked for it ^^