Anything I want to regularly reread, especially long series and/or books, I want in DRM-free ePUB ebook so I can read it wherever and whenever I want.

I still carry a few mass-market trim size paperbacks on trips, because I like to fall asleep reading and if I drop my reading material on the floor I don't want it to crack.

Nonfiction I'm often getting from the library, usually in ebook format these days as well.