I've found that if I read before falling asleep - regardless of subject, difficulty or how closely I focus - I tend to have vivid dreams that I actually remember. There doesn't seem to be any relation between the book and dream, but if I don't read I tend not to dream at all (or at least I don't have any recollection of it). I only use an e-reader so it's not even like it's a tactile response to traditional books or anything like that. Outside of that reason, I enjoy reading books but I don't generally do so recreationally since I usually feel like I could be "multitasking" doing other stuff instead.