Over the years, there have been a few different books which have inspired me; surprisingly, some of them weren't books specifically geared toward GMing.
Dungeonscape for D&D 3rd Edition was one such book. It came late in the life of 3rd Edition, and it was mainly geared toward building dungeons and traps, but it opened my eyes to a lot of things I had never considered before. For example, I had -until that point- mostly viewed traps as a small piece of a room. Some of the concepts in that book pushed me to think outside the box more; utilize the idea that the dungeon as a whole could function as connected pieces rather than thinking in terms of one room at a time. To some extent, I was already doing that, but I didn't fully realize how to implement some of my ideas in the context of a rpg. While I wouldn't say Dungeonscape was necessarily a great GMing resource it -for me personally- prompted a different way of looking at certain things.
Somewhere along the line, I picked up a book called GM-Fu. I'm of the impression it was part of a seminar held during Gencon 2008. It's only a 30 page booklet, but it contains some creativity exercises which helped me get better at story telling.
GURPS Horror for GURPS 4th Edition is a book which I highly value. While it is geared toward a particular genre, there are chapters which discuss different types of fear and how small details such as monster choice can have a big impact on the feel and tone of a game. A lot of the things learned from that book are things I believe can be applied to games in general.
Really, I think some of the best learning tools I've had access to have been some of the people I've gamed with. I've had the good fortune of being exposed to a lot of good GMs. None of them were perfect, but most of them had traits which I feel made my experience as a player better. From them, I learned good habits which have helped me along the way.
As nice as it was to have good GMs, I think it's fair to say that some of the lessons I learned which stuck the most were the horrible and really bad GMs I had the misfortune of playing with. As a player, there were things I was subjected to that weren't fun, and I remembered those times. By comparing and contrasting them with the good times I had, I developed a good sense of my likes and dislikes when it came to gaming, and I also learned what my own strengths and weaknesses were when it came to running a game.
Beyond all of that, the best resource I have would have to be the players I run games for. They are the most direct feedback line I have to things I do or don't do. Reading their reactions to things have lead me to learn what I could do better at. A little bit of communication and empathy can go a long way.
