SSI Open Water Diver. Not because the book is particularly (or at all) well written, but because it was my entry into an amazing hobby -- SCUBA diving. It's an enjoyable activity on its own, and the training model for SCUBA is actually a good way to teach a lot of things to consumers (a VC friend who just picked up diving too believes this as well). For actual book as book, probably "First In; How Seven CIA Officers Opened the War on Terror in Afghanistan" by Gary Schroen. They basically went in with $30mm or so and an old helicopter, 2 SF ODAs, and accomplished most of the good accomplished to date in Afghanistan within 2 months. For fiction, I'd love to say Neal Stephenson's Baroque Cycle, but it's just too fucking long. He needs an editor; I'm on book 3. For unexpectedly awesome, Suarez's Daemon and Freedom.
Also recommended in this reply