The Ashley Book of Knots (41.1MB) is the bible when it comes to the subject. It is a beast of a book with over 600 pages and can be quite intimidating for someone just starting out compared to some others with more mainstream information.
From the wiki site: It is an encyclopedia of knots written and illustrated by the American artist Clifford W. Ashley. First published in 1944, it was the culmination of over 11 years of work. The book contains more than 3800 numbered entries and an estimated 7000 illustrations. The entries include knot instructions, uses, and some histories, categorized by type or function. It remains one of the most important and comprehensive books on knots.
You can find it on the used market at Amazon but they tend to be more than $30 and conditions will vary greatly. If you do a Google search, it is available in .pdf format from various locations. The version we have here allows you to open it in Adobe and do a search, which is great for such a large work.