The Ashley Book of Knots by Clifford W. Ashley is one of the best and most well known knot reference books. The subtitle “Every Practical Knot – What it Looks Like, Who Uses it, Where it Comes From, and How to Tie It” pretty much says it all. The book contains drawings and instructions for thousands of different types of knots. It starts with the humble Sheet Bend and ends with the Crossing Knot with many useful and obscure knots along the way, as well as splicing, matting and other related subjects. Full of anecdotes and historical footnotes, this is a profoundly good read. The book is so important it even has a Wikipedia entry. There’s a limited preview on Google Books should you still be on the fence. New price is about $55.00 on Amazon (it’s a big hardback) and used prices start around $35.00.