"Hardcore Zen" and "Sit Down and Shut Up", both by Brad Warner.
Their influence isn't just due to their coherent and appealing explanation of Zen meditation. I'd been practicing Zen for years before I read them.
No, what these books really offered me is a great example of getting to the real heart of an issue by throwing away the dogma and looking at the situation with fresh eyes. So often when we're faced with an unsolvable problem, it's because some of our lower-level assumptions make the problem unsolvable.
With teaching Zen, Warner realized that the dogma isn't the point. Freed from the typical methods of teaching Zen, he came up with an even cleaner and more elegant method of doing so. He tossed the bathwater and kept the baby.
Every time I've overcome a major, seemingly intractable snag in a program, it's because of a question I wasn't asking myself that I should have. And I wasn't asking the question because I'd already assumed an answer instead of actually seeing the problem and formulating it.
Both Warner's example and the practice of Zen meditation showed me how to throw out my assumptions and build a problem space entirely from scratch.