The Fabric of the Cosmos
By Brian Greene. I've read it cover to cover three times and have re-read certain sections probably three more times. Just thinking about it makes me want to read it again. I've got it in hardcopy and eBook.
In the fiction world, there are a handful of books I've read a couple of times but one that jumps out at me is Niven's The Legacy of Heorot. I love every page of that book. He penned a sequel, however, that I found intensely disappointing.