This is what I do. First of all, I have a ton of books, mostly purchased at yard sales andthrift stores. So, when switching gears I bring all the related books up from our shelves in the basement, and switch them out with the books we had up here. It keeps the reading interesting and they don't have to look at the same books. They are welcome to go down stairs and look- but we all know that is a lot of work! ;)
Also, I have them help me in this process. I only have three kids, so they each get to choose pretty much an entire shelf, if they want to.
The other thing is, we keep shelves in their rooms. If they have a series that THEY like, personally, then it goes in their room. Or if they just want to keep certain books in their, it's fine.