Here’s a great textbook about how to teach and illustrate principles of economics. If you browse through it, you may notice that it is not merely a textbook but a compendium of tips from professors about how to illustrate what economics is all about. It also makes for interesting reading (probably more than the typical textbook).
I wonder if these group-written books about teaching exist for other disciplines: namely literature, history, writing. Wikis probably could fulfill that function admirably. In this case, one professor seems to have compiled and edited everything. (There is a PDF of the complete online book).