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.
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.