From Rockford Lhotka's Expert C# 2008 and VB 2008 Business Objects books
Project <-> Resource is actually a M:M relationship, so ProjectTracker already demonstrates how to implement M:M, as well as 1:M.
I have yet to see a case where, in an OO design, there's a real argument for M:M like there is in the relational world. Since objects don't reflect just data, the nature of the relationships comes into play, and the relationships in a data-oriented M:M concept are (in my experience) never symmetric, and so the objects are never the same.
As I've said many times in the past, ProjectTracker is the demo app for the Expert Business Objects book. Therefore it has to meet certain requirements, not least of which is that I have to be able to describe the app (design and implementation) in just a couple chapters. That severely limits its size/complexity.
If I followed a different model, such as creating a more complex sample app that comes with the purchase of an ebook or video series focused on that app, that would break me out of the limitations of a paper book.
Or I could just create a large and complex sample app and give it away - but at the end of the day, like you, I need to generate income :)
Rocky