Since I learned on my own years ago, I would highly second Bob's advice and get Byrne's books, starting with the Standard Book of Pool and Billiards. Not only is it excellent for Pool but as he said the second half is completely on three cushion and very well written.
I still have my marked up copy (as well as the Trick Shot, Advanced, and Wonderful World copies) that I used to learn 3 cushion. I went chapter to chapter, shot to shot, and made marks when I made the shot and went back again and reread many times and made a second notation years later when I fully understood the concept and shot category.
The systems books are nice (I have all of Walt Harris's and Eddie Robin's book), have read some of Ceulemans book and might get Bill Smith's book when we finally get a table in the area and I can start playing again. Frankly I picked up quite a few gems from Walt's books, but they are sometimes complex and/or situational and you won't learn the overall game as well from them. But there are some very cool systems buried among the 4 or 5 volumes...
Scott