Using the Eclipse Framework

I saw there is a section on developing plugins, but I was unable to determine from the information on Amazon whether "Eclipse in Action" has any information on using/extending the actual Eclipse framework itself.

I'm not sure I understand the question. Writing plugins is the way that you extend the framework.
Eclipse in Action has two chapters on plugins. The first one, chapter 8, is an introduction. Chapter 9 is about twice as long and walks through the code for a good sized plugin that does log4j integration into Eclipse. There is also an appendix on SWT and one on JFace programming. The bulk of the book though is on using Eclipse as an effective tool for general Java development.
If you mean using bits of the framework in your own non-IDE applications then no, it doesn't go into that per se. However the SWT and JFace material would still be applicable if it's a GUI application, and the two plugin chapters would still be applicable if your application was built using the plugin concept.