I'm a big fan of Howard Hughes' book "Once Upon A Time in the Italian West", which I think is a must have for any self-respecting fan of the genre.
Using that as my benchmark, I was initially a little underwhelmed and disappointed in "10,00 Ways to Die" on a first read-through. The two authors employ very different approaches. Hughes looks at a core set of films in great detail and uses that framework to comment briefly and in passing on a wider range of related films including sequels. The Cox book, on the other hand, looks at a much wider range of films, including such neglected works as "Arizona Colt", Requiescant", "Johnny Hamlet", "Bandidos" and "The Price Of Power" that Hughes ignores, but generally covers them in less detail.
Hughes is very good on aspects such the inspiration for the films, the actors, the locations and the music. He is also pretty even-handed in his approach. Cox has little to say about the music (he admits he has a tin ear for a good tune) and is much more partisan - which is great as I like opinionated authors. But on the other hand, Cox brings his perspective both as a film maker and also as an enthusiast to the genre and his book is full of fascinating observations that you won't find anywhere else. My only complaint is that the ending of the book seems to me to be rather abrupt, almost as if Cox had a hard page limit imposed by his publishers and he realised that he was fast approaching it!
Conclusion? The more I read "10,000 Ways to Die" the more it grows on me. Cox has great enthusiasm for the genre (as has Hughes) and a quirky yet very readable and informative writing style. Ultimately, I think you need both books and I'm glad I have them.