Cities in Flight by James Blish
A strangely well-written book for being such hard science fiction.
This book described badly: the entire city of New York realizes its ultimate destiny as an intergalactic tech nomad hipster douche constantly sermonizing about quantum woo. In the end, universe gets remade in the image of a good old boy from Brooklyn.
Major hilarious downside: I think I have discovered the origin of "As you know, Bob..." That infamous science fiction exposition technique. The amount of Bobs having things explained to them in this book is unreal. (It's only two Bobs, but that's two too many. Am I right?) Read for a laugh.
At least four hundred billion stars. Not a waste of time.