And for a cynical take on the "modern engineer brings advances to middle age" also taking into account social aspects, Mark Twain's "A Connecticut Yankee in King Arthur's Court" (http://www.gutenberg.org/ebooks/86) is good reading.
Don't forget "The man who came early", which deals with just the sort of cannot-reconstruct-the-technology-chain issue in case of accidental time travel.
And of course the handy Time Traveler's Poster from Topatoco ...