Hi Dave,
There is a single chapter at the end of the book dedicated to the art of plugin development itself. It walks the reader through the creation of a typical plugin and illustrates how to use the hooks that Rails provides and some Ruby metaprogramming practices to graft new features on to Rails itself. This isn't really the focus of the book (which is on using pre-existing plugins), but we felt it was important to at least address it, and also talk about
testing and distributing your plugin code as well.
Although its existence is addressed, the Rails Engines plugin is not used specifically in any of the projects. We had a really tough time of narrowing down the list of plugins we started with to those that felt most practical for beginning to intermediate Rails developers, so it's not that we don't think it's an important one
.