Here you'll find some of the best manuals out there, for beginners as well as for more advanced operators.
1. The Official User Guide by Teenage Engineering
2. The Complete Teenage Engineering PO-33 K.O! Guide - by Callum Howkins
3. Tips and tricks video - by Gar Hoover (check out his other awesome videos here)
4. Hidden Features and Extended Manual - a very good list of features not covered by the official manual.