I found Keel's work here to be quite enjoyable. As the title suggests, this novel is a compendium of a vast variety of topics across the broad spectrum of the paranormal.
On the positive side, I found his work very readable. It covers a wide variety of material, so chances are you will hit upon something new. The rather terse nature of the writing kept it from getting bogged down in any one topic.
I think that the positive sides above can also be downsides, depending on what you're expecting from the book. First, this is one of those works that attempts to cover a very broad spectrum of material. As such, each individual topic is covered in a fairly brief and shallow manner. Don't look to this book for particular detail on any one topic...there are other books which do a much better job of specializing on each. Most accounts of a sighting or incident cover little more than a sentence, or the occasional paragraph. Accounts are included in bulk but in brief, to give credibility to a topic by mass, rather than to give detail.
Also, I agree with previous reviewers that Keel is far from objective in his writing. He certainly offers his own (sometimes slightly outlandish) explanations for events and creatures. Those who would be offended by such a writing style may have issues here. Personally, I have no problem taking opinions with a grain of salt, and forming my own conclusions from the factual material (which is by far the majority of the text). I did not find Keel's opinions overbearing, and the vast remainder is enjoyable and factual.
In brief, the book is good, and I'd recommend it. It's best suited for those looking for a tour of the broader spectrum of the paranormal: The material is often brief and jumpy, but detail is not the object of this title. What it lacks in focus, it makes up for in variety.