I'm another pro Animated Knots guy. When ever I refer someone to a particular knot, I reference the link to Animated Knots. They do have a apps for the popular smartphones too.
But when it comes to a paper book, I a really like Knots, Splices and Fancy Work by Chas L. Spencer. My copy is a hand me down from my father but you can occasionally find it online at some of the used book places. It's a good one.