Knots are pretty easy for me: I do fine with book diagrams, usually. (Sometimes not -- if the book diagram is just a rote path without making the knots geometry visible, that doesn't work well for me.) Of course, I then need to practice it enough that I can do it by muscle memory alone -- after all, "What other way would it go together?" isn't going to be much help when you're altitude-stupid or exhaustion-stupid.

The site I've seen people rec is animated knots. It's not a complete source for climbing knots, but a solid beginning. (Some of the classic climbing knots are under "basic", not "climbing".) Mountaineering: The Freedom of the Hills is fairly comprehensive, and what I learned from; I don't know if there's an online site that matches it.