Now that you've finished this tutorial and tried out all the techniques presented, hopefully you're eager to combine all the objects with each other and make your own discoveries - after all, that's exactly the intention behind Pd's design.
There is, of course, virtually no end to further study of the concepts presented here. Many other books about digital sound processing await your perusal - especially "Theory and Techniques of Electronic Music" by Pd's main designer Miller Puckette. It would certainly be advisable to further increase your proficiency with and knowledge of acoustics, studio recording techniques, and programming. You may also want to consider acquiring fluency with the basics of a sequencing program for programming sound. However, enthusiasm, artistic satisfaction, and aesthetic reflection are still the most important things.
For any further questions about Pd, I encourage you to contact the "Pd-list" Pd community. This book was written according to Pd version 0.39 from late 2007. Hopefully the information contained here will not be outdated too quickly.