This series uses example-based tutorials to explain and illustrate concepts and new lessons build upon concepts learned from previous lessons and examples. By the time you've gone through one of the books the most fundamental lessons have been iterated over numerous times (but without beating you over the head with them), and that's how these things stick. But honestly, my favorite things about these books are that they're priced to move (free & cheap, based on what format you're looking for) and that they're open-source (the source code is up on Zed's Gitorious account) but they're edited; no wild-ass Wikipedia style misinformation, just people contributing what they know where they think it'll do some good.

Long story short, it's inexpensive ($2.99 e-book, $7.99 dead tree!), well written (as well written as the beloved Why's (Poignant) Guide to Ruby), and fun in a nerdy sort of "taught myself calculus one summer back in high school" sort of way. Did you teach yourself calculus one summer back in high school? If you did, this book is probably beneath you. But since I spent my summers in high school at the Warped Tour, working dead-end food-service jobs, and swimming in creeks, I got a lot out of a surprisingly thin book.

If you're interested in a gentle introduction to programming, looking for a refresher on how computers do "The Maths", or just want to contribute a few dollars towards a noble cause (that is, convincing Steve to write volume 2) this is a worthwhile purchase. Double word score bonus to the fact that unless we have a massive quantum compute breakthrough within our lifetimes, the contents of this book should withstand factual atrophy astonishingly well.