I've got the Android app and love it, as well as Knots 3D.
Most knot enthusiasts will already know about it, but in the analog world The Ashley Book of Knots is fantastic. Beautifully illustrated; the author, Clifford Ashley, was a marine painter and spent decades documenting almost 4,000 knots.
It is a wonderful catalog, but I don't find it to be as useful for learning to tie the actual knots as books by Budworth, Pawson, etc. (or Youtube, these days)
One of the cool aspects of knots for this audience is that they have a unix-like aspect where multiple individually-useful knots can be "piped" together, the example I like to use is the Trucker's Hitch:
https://www.animatedknots.com/truckers-hitch-knot
(which also can be tied in multiple ways, depending on which knots you combine to make it)
> The Ashley Book of Knots is fantastic.
Yup. Referring to knots by their ABoK numbers is also more practical than by their wildly varying names.