IMO one of the best books every GM should read is Your Best Game Ever. There are a lot of gems there that experienced GMs picked along the way, and even if you're experienced there is bound to be something interesting. One thing that I always remember from that book are tips on how to make your NPCs more memorable by giving them quirks like stutter or have a manerism when speaking.

Another cool upcoming book is The Weird, it's still not available even for backers but it's in the final stages so it should be available soon, but there is a preview available to see what to expect.