The book is "Black Holes and Timewarps: Einstein's Outrageous Legacy" by Kip Thorne. I, too, highly recommend it. The majority of it serves to give the reader a decent grasp on General Relativity, and the concept of black holes. Towards the end, Thorne raises wild possibilites about time travel, wormholes, and the construction and usage of such. This portion he readily admits in the text to be fanciful at best, but the speculation is fun.