I used to work in a bookstore, and I've been working in libraries almost my entire career. Most books have no value. I've probably thrown out a million books in my life; most of them have been diet books, cook books, and political biographies.
My current library is around 2000 square feet and I acquire around 1000 books a year, so I have to toss around 1000 books a year, because they're made of matter and take up space.
It's a bit less of a thing than it used to be, but disposable books on technology were a thing for quite a while too. Think titles like "iPhone 6 for Dummies", "Learn Flash in 24 Hours", or "Windows 8 for Seniors" - there are a lot of books which were written (usually on the cheap) for a specific audience at a specific time, and which have no enduring value.
Also along these lines: test prep and study guide books. Same deal really.
My current library is around 2000 square feet and I acquire around 1000 books a year, so I have to toss around 1000 books a year, because they're made of matter and take up space.