I've been (slowly) writing a book for about a year and a half now probably, and Pandoc has made it so easy. I write everything in Markdown and use a seven-ish line Makefile and that's it. It generates the PDF and EPUB both extremely well and the customizability of everything in the generation process is fantastic.

I end up using it for random doc generation too in my day to day. It's just a damn fine piece of software that always works and has every feature I could dream up for my use cases.