Fittings are parametric parts. That's a whole new world of AutoCAD MEP content customization and can be very complex. Depending on what you're wanting to do, you might only have to edit the existing catalog of parameters to get your desired results.
Might I suggest this book as well: http://www.amazon.com/Mastering-Auto.../dp/1439057664
If you're really wanting to dive in deep with AutoCAD MEP, then that's the book that will get you where you want sooner than any other book available.