Only technical books, I still prefer paper books for some references, especially if they also come with an ebook version -- some publishers give you the PDF if you buy the paper book.
As for novels and similar long-form text, I can't stand paper books anymore and every time I buy one (when the used bookstore Sirens lure me in) I end up regretting it and buying the ebook if I decide to actually read it.
EDIT: Oh, and sheet music books, but I don't really consider those "books" -- but I have a huge stack given that I play three instruments, so each requires its own stack of books.