It's a coffee-table book, but one that is very easy to read, and comes with plenty of gorgeous pictures covering the existing Colosseum structure. I particularly enjoyed the beautiful architectural drawings between pages 14 and 17 -- the Colosseum at its imagined peak. It won't shake up your intellectual foundations, but it was certainly a worthy purchase from the Colosseum's gift shop, and my Ancient History students find it a very useful resource.
Decent photo book with brief history of the use and structure of the Colosseum in Rome. Purchased it in Rome, so it does serve as a nice memory. The structure in person is pretty awe inspiring.