Well, there aren't a lot of frills, bells, or whistles in the title, or a lot of mystery for that matter. Travels with Charley: In Search of America is just what the title says it is: the story of John Steinbeck's trip through the U.S. with his dog as the silent witness (and sometimes counterpoint) to everything he sees.
As we've noted in the "Characters" and "Symbolism" sections, Charley's unique qualities as a silent companion and nearly-human dog confer more importance on him than you might expect in a dog, so it's only fair that he appears in the title as well.
You'll notice, though, that it's not called My Travels with Charley. Steinbeck leaves himself out of the title, which seems appropriate given his commitment to kind of anonymously observing everything around him as he goes.