Either Reynold’s Eversion or Adrian Tchaikovsky’s Children of Time.

If you liked the “harder sci fi” type feel to it, then my recommendations are:

Tau Zero (Anderson), or Rendezvous with Rama (Clark)

Or maybe the Xeelee sequence (Baxter). Start with Raft, Timelike Infinite, Flux, and Ring – then take a break.

Outside the box recommendation: Pirenesi by Clarke – about 300 pages – I’m not giving spoilers.