 Kevin Yee’s book Walt Disney World Hidden History (second edition, 2014). This softcover book tracks Disney “hidden history” — remnants of former attractions and tributes to Imagineers as well as other Disney officials — hiding in plain sight in the parks. There are over 300 entries in the book!
Whether tribute or remnant, each item discussed starts with something visible in today’s parks; the idea is that this is something you could visit and see with your own eyes, and then appreciate the historical thinking behind it. Since most visitors aren’t local, the book includes a photo of every item discussed so you’ll know what to look for.