There are many many free and legal books available, so it's not especially interesting to point to just one. What's interesting is to know which of them are worth reading (and, preferably, why).
I'm quite prepared to read something when someone I know recomends it or when someone I don't recomends it and gives reasons. But there are a whole load of choices out there so I see no reason why I'd pick this one. 30 pages isn't long but it's also not short.