This book might be a little dated but it's still a good read:
"Pioneering the Space Frontier", Bantam Books, 1986.
It is a report by a panel of space experts and astronauts appointed by Ronald Reagan to chart the future of space exploration. Interestingly enough it was released after the Challenger accident.
Even though it's a little old, it is still one of the best, most in depth books I've read about what could be done over the next 50 years. It goes into a lot of detail about advancing science, exploring, prospecting, and settling the solar system. It begins with 9 rationales for space exploration. The concepts in the book remain valid today, it's just that the shuttle won't be the vehicle used.