What book have you read that just completely blew your mind in terms of the author's imagination? What book made you think, "Wow, I could never come up with something like that!"?
I would say the most difficult book I have ever read, in terms of imagination, would be "The Hitchhiker's Guide to the Galaxy" by Douglas Adams. The book is just so out there and imaginative that it is hard to wrap your head around it at times.