We all need to learn, and sometimes a book is our preferred method. Should questions asking for book recommendations be on topic?
Questions about "which book" have the same issues as product recommendation questions. Mainly that there are multiple correct answers, they attract spam and people accidentally getting flagged for spam, they are regional, change over time, and opinion based. This would be better asked in the chat room, or to simply compare the product reviews from an online retailer.
Sometimes books are timeless. Some techniques written in those books will last longer than this site probably will. With that in mind, I agree that book recommendations could encourage spam. I'm wondering if this would be something which could be worked into a blog. Such as, someone buys a book, finds it useful, then is willing to write a blog entry about the book based on its merits. It would keep it off the main site, but allow the information to be known. If someone found a book to be particularly helpful or non-helpful, it would go in here. A blog could be used for a lot more than that, as well. I know some episodes of TV DIY shows could get reviewed, or in particular a write-up about how to do things which wouldn't really fit in with the SE Q&A model.
A book is a tool, meaning a device that helps you accomplish a task. If someone is looking for know-how on how to get started with home improvement projects then a good book on the subject is of a huge value.
Comparing product reviews from online sellers is iffy, people get paid to write and spam those too.
If worried about spam you deal with it as it comes up. If no one asked for a book suggestion and someone posts one then you can pretty much call it spam. Saying you won't do item X becuase of item Y strikes me as a bit silly.