Basically, though, it's the story of elephants and their relatives. The (so far) to me most interesting insight from the book is that probiscidean radiation appears to have come in three waves: the first wave being "primitive" probiscideans, including deinotheres and mastodons, a second wave of the paraphylitic gomphotheres, and the final wave of stegodonts and real elephants (including the mammoth.) That's of course an over-simplification, but more or less true nonetheless.
The interesting thing to me is the survival of several "primitive" branches of the proboscidean tree until very late times. The true elephants (including the mammoth) are all late appearing animals, so that they survive today isn't particularly surprising. Nor is it surprising that their close relatives the stegodonts still survived. The fact that gomphotheres were still going strong in South America as recently as 10,000 years ago (or possibly even later) as well as the American mastodon lasting as long is it did. Heck, even the bizarre "hoe-tusker" Deinotherium seems to have lasted until only a million years ago.
Anyway, I find the notion of these animals that we just missed being able to see living a very compelling and frustrating and sad thought. For some reason the mighty probiscideans have been a lot on my mind, too. Luckily, we've got some really good artists out there who know how to give us a very nice view of what these animals may have looked like; I've attached some such artwork to various other posts, but here one of Cuvieronius, an "elephant" that paleo-indians seem to have fed on in what is today Latin America.