I've been working with HAProxy a lot recently. Their documentation has the dubious honour of being the most man-page-like of any man pages I've ever seen. The problem with man pages is that they're perfectly clear after you understand them ... which is a bit of a catch-22. They're useless for learning from, and this has never been more true than with the HAProxy documentation. Don't get me wrong: it's extensive, it's accurate ... and it's still completely useless if you don't already know what you're doing.
I turned to Nick Ramirez's Load Balancing with HAProxy (2016). It was the only choice (in late 2016 there were NO other books available on the topic), which made it doubly lucky that it's actually a very good choice. Ramirez understands something that a lot of authors of technical books don't get: you start with the simple stuff, you explain it with plain words, and you advance through examples that make sense with the student's current knowledge. It sounds simple and obvious, but I've read a lot of technical books that started in the middle, used unnecessarily florid language, gave examples where they had to say "oh, you'll understand that bit later," or went out of sequence to show you something more interesting early on - with the only possible end result being confusion.
HAProxy's configs make sense, but they're distinctly opaque until you've got a good explanation to read. And this is the help you need: this is a cleanly written book, beautifully structured to build from chapter to chapter on the knowledge you've already acquired, and exactly what you'll need to learn how to configure HAProxy. Highly recommended.