Cities in Flight

This is a long book, but absolutely fantastic. It redefined for me the scale at which science fiction was possible, especially given the human elements are very fleshed out (as opposed to other massive space epics, like Olaf Stapledon's 'Last and First Men'). A brilliant look at the future, going off of only two small changes - what if we had drugs to defeat death, and cities could fly. - @RichardLitt