Art of Knotting and Splicing United States Naval Institute
If you can get hold of this book, I would highly recommend it! It is packed full of practical naval knots, plus a few decorative additions that sailors may have used in the past. Each Knot has a detailed description on how to tie it, and also a series of photographs to accompany those instructions. I also like the fact that they are photographs, because sometimes illustrations are used, the artist will sometimes make the Knot look better than it actually is.
I think that is well worth checking Amazon and eBay on a regular basis to see if anyone is selling it off cheap.