I also use the following apps to read on the playbook:
Kobo (For Kobo purchase books)
Pdf Reader (by Sporadic Software)
Overdrive Media (for Library books)
After I buy a book I immediately clean it and download it into Calibre. Then I either copy the book into the Playbook via Z drive (when attached to the PC), or use SugarSync or Dropbox or whatever to download it to PC when I'm ready to read it.
Enjoy!!