All you need is some rope and some knowhow
That one is the holy grail of ropeworking books. I have yet to get my hands on it so I have never seen it but everyone serious about the hobby loves it.
That is the creative ropecraft. The illustrations can be a bit difficult but its a great beginner book.
This is one of Des Pawson's books. I borrowed it from a friend once, tons of info. I had to give it back though. ><
This is a nice cheap book too, I have never seen it myself but it is one that is recommended a lot on various youtube ropecraft channels.
> low investment
Hah! Yeah sure, until one day you wake up and realize you have like a mile of paracord and 100 pounds of beads, buckles, and other accessories.
Also to add to this, beyond just knots is macrame and all kinds of useful and decorative stuff. If you look around you can find a PDF of the Ashley Book of Knots. I don't think I have ever been into a Half-Priced Books store that did not have at least a dozen knot books. Look in the section where the boat books are.
 - https://www.amazon.com/Ashley-Book-Knots-Clifford-W/dp/03850...