This UFO book has several different authors, but is edited by one Charles Bowen, who I think was also once the editor of Flying Saucer Review, a British UFO magazine.
The content of the book is a mixed bag. In one chapter, there are literally dozens of reports of UFO landings and sightings of entities from the ufologically-famous French "flap" of 1954. Whilst this is interesting, it does not go into any great detail about any of these, so there is a tendency for one's eyes to glaze over after a while, even if you are interested in the subject.
In other chapters, there is some interesting statistical analysis, and a more in-depth look at some classic UFO cases, such as the famous "AVB" case, and the case of Barney and Betty Hill. Another interesting thing about this book is that it gets "off the beaten track" giving some details on some little-known UFO cases, which you are unlikely to have read about in any other book.
Anyone buying this book should be aware that it is now almost half a century old, so it is very dated. Please bear this in mind. Nonetheless, if you are interested in the subject, it is maybe worth a look.