I know a fair bit about mustangs and about equestrian history, and I went into this book hoping it would be a summary of mustangs in the American imagination as well as a thorough overview of the current mustang debate. I didn't really find either.
In general, this was a well-written and thoughtful book. It was an enjoyable read that moved right along, and the last few chapters did give a good snapshot of some of the challenges that mustangs face in the American west today (without going into too much depth, alas). Stillman was best when handling contemporary issues, and telling stories of people that she'd met or interacted with. Her background in journalism served her well.
However, the overall framing and narrative of the book had some major flaws. First and foremost was that Stillman made no attempt to define mustang. She did not give any boundaries to what she would be discussing, and did not attempt to parse out the nuances between range-bred cavalry horses, range-bred ranch horses, semi-managed herds, or truly feral herds. There is a world of difference between a horse that is part of a semi-managed group that belongs to the US Cavalry or a particular ranch and a horse that exists completely untouched within the larger ecosystem out west. Not every horse that lived in the American west was a mustang; not even every horse roaming loose in the west was a mustang, in a time when loose herd management was more typical. (Did they intermingle? Probably. I'm not aware of any good research that looks more closely at those boundaries, though.)
This problem leads into the second major flaw, which is that Stillman's book then becomes a history not of the mustang but of the horse in the American west. She considers US Cavalry remounts such as Comanche, Spanish conquistador's horses, cow ponies on cattle drives, Native American herds, Wild Bill Hickock's show horses, and others to be part and parcel of the mustang story. Some of them may be - but most of them are not. Many times, the story became more about the people involved than the horses. Granted, it's difficult to truly write the history of a horse - who after all don't communicate or write down their own history - but the overall book felt more like a chronicling of the things humans did in the American west, and oh by the way they usually involved horses.
The good news is that this was not a bad book, and does add to the literature about mustangs, so it's not a loss. It's simply a missed opportunity.