I know you asked for specific sources, but this video does an overview of training resources for DP.
https://vimeo.com/86649594
For basic stuff, there is a link on the startup page for DP labelled "DP Tutorial and Demo Files". That will lead you to a few simple project files that show some basics.
There is also a WIKI that has about 50 simple illustrations of DP functions.
http://en.wikiaudio.org/Category:Digital_Performer
You should go through the sections of the Getting Started manual that are relevant to your use of sequencers and your equipment if you haven't already. Getting the basic setup right at the start prevents a lot of downstream mysteries.
Look at the Tips and Trick thread on this forum. Early in that thread, Shooshie posted a lot of stuff you need to look at to make the best use of MIDI tools in DP.
One paid source you might want to look at is Eli Krantzberg's overview of DP at Groove3. These are about version 8, but just about all of it is useful for learning 8 or 9. If you sign up for a month of unlimited access at Groove3 for about $10, you'll be able to work through his two series on DP basics and DP advanced, and another set of videos on Version 9 changes.
Once you're familiar with some basics, it can be useful to go back and read some sections of the manual. When you know nothing about DP, the manual is like a wall of hieroglyphics, but once you have some basic experience, it makes perfect sense and is worth the time.