I had a chance to flip through Arva Moore Parks' book George Merrick's Coral Gables "Where Your 'Castles in Spain' Are Made Real!" (Centennial Press, 2006), and it looks like something worth sitting down and reading.
Another interesting book is Coral Gables: Miami Riviera - An Architectural Guide, which has walking tours, maps, and photos.
Speaking of photos, the website Coral Gables Memory has piles of old photographs. Enjoy!