People have already mentioned my two suggestions: 4E DMG2 and The Lazy Dungeon Master.

This isn't a specific book recommendation, but I would recommend reading a diverse range of RPG books, across a breadth of topics which interest you. In particular, reading the storytelling sections and how those guidelines interact with the delivery mechanics of the specific game can offer a new perspective on how to run your games.

For example, I enjoy horror roleplaying in particular, so books I may read are Call of Cthulhu for Cosmic Horror or the World of Darkness/Chronicles of Darkness books for Personal Horror. While I run Call of Cthulhu a lot more than World of Darkness, the tips on storytelling personal conflict from WoD offer some interesting insight into running sanity conflicts in CoC.