Shift Omnibus Edition (Wool, #6-8) by Hugh Howey
I read this straight after the Wool series. The Shift trilogy fills in the back story of how the silos were created and then starts to overlap with the events from Wool. Shift didn't have quite the same tension as Wool and I didn't care about the characters as much. Still hooked though and can't wait for Dust later this year...
Wool Omnibus (Wool, #1-5) by Hugh Howey
Collection of five novellas set in a mysterious silo. A little more is revealed with each chapter and the series gets more compelling and ambiguous as we learn more about what's really going on. Didn't know what to expect (got this as a Christmas Present) but very enjoyable and I've gone straight on to the next omnibus with a final tranche due later this year.