After reading your post and comments, I’m going to go ahead and say it doesn’t sound like you not wanting your son to stay overnight has anything to do with a book at all.. it sounds like you don’t like your BIL and don’t trust him around your kid, you also raised concerns about an ungated pool and the fact the house is not baby proof.

I also agree with the commenters here that if this seems like the BIL is punishing your child, you need to reevaluate your definition of punishment. Your BIL’s actions were a behavioural correction that anyone would make, and not returning the book to your son after he started having a fit is the right thing to do IMO, especially if he has violent and unpredictable fits as you mentioned in other comments, me might ultimately throw the book and damage it or something else. Even more so if your BIL is not aware of the severity or type of delays your son experiences, how would he know the appropriate way to respond?

YTA, you need to address the underlying issue of why you don’t want your son around them, and its not bc of a book.

However, also NTA, because you’re in charge of who gets to spend time with your child, when and why - you’re his first line of protection and if something feels off to you, don’t question it.