I believe these are most (if not all?) of the official books, so hopefully this list is helpful while searching the library:
- Fun with a Pencil (1939).
- Figure Drawing for All It’s Worth (1943).
- Creative Illustration (1947).
- Successful Drawing (1951).
- Drawing the Head and Hands (1956).
- The Eye of the Painter (1961).
- I’d Love to Draw! (2014).
They don’t have to be read in any particular order, so if one of them seems more appealing to you, I’d say go for it! And there are other people who’ve written about the method both online and in other books, but if you can find the originals, that’d be best. :3
The search term ‘Andrew Loomis Collection’ might also be helpful? It was at my rinkydink highschool library, but I’m not sure if they’re collated elsewhere too.