I'll kick it off with a couple:
The Paul Gravett edited comics instalment in the unlovable 1001 ... Before you Die series is a surprisingly excellent, serious minded attempt to represent the best of the comics medium. Its purview starts before the twentieth century and it covers comics in an admirable number of international traditions. There is much here that is recondite and wholly new to me. Also taking a road less travelled for Anglophone readers is The Essential Guide to World Comics edited by Tim Pilcher and Brad Brooks.