I'd be interested to hear what people choose for their "other" books...the one that's not exactly on topic, but has relevant skills or metaphoric skills. (Genius Code, p21, "Selecting your books").
For example, for birdwatching, I added a book about learning how to draw, on the assumption that perspective and detail and representation would be relevant skills to identifying field marks.
I sometimes feel like it's a stretch. Eg, learning a programming language, what would a good "other" book be?