Development
Read the README!
If you’re interested in developing plugins that ultilise the engines plugin, the first thing you need to read is the engines plugin README, which lays down all the basics about how it enhances Rails’ own plugin mechanism.
Tutorials
Gwyn Morfey has written a guide building plugins that can take advantage of the engines plugin’s features.
Joe at AlterLabs also wrote one of the best introductions to developing an engine (thought this has not been updated to reflect the 1.2 release of the engines plugin yet).