If you can pick up the Locomotive Cyclopedia Vol 1 & 2 from Kalmbach, vol 1 is on steam engines, and vol 2 is on diesels. Both were put together by Lynn Westcott, so they date back to the 70's, but I think they are still in print. The first chapter of each book explains in vol 1 how a steam engine works and what the various parts are, vol 2 chapter 1 explains how a diesel engine works and nomenclature for parts.