He's apparently a novelist and author of nonfiction, who has written for such questionable projects as the History Channel. He has no discernable scholarly credentials.

The book in question does not appear to cite any sources; it's not a scholarly work; it is, as its original title ("The Hitler Fact Book") implies, simple a list of largely unrelated factoids, as befits the work of an author who used to write for "Ripley's Believe It or Not."

The book has never been reviewed or mentioned by any of the respected scholarly journals indexed by JSTOR.

None of this is to say that the book is unreliable, but it doesn't have many features that would suggest that it IS reliable, and if you are writing something and are looking for a source to cite that will make your essay look reliable, I'm sure you can find something much better than this.

It's basically "infotainment, " not scholarship. It may all be perfectly true, but there's no way for you to be sure of that without conducting your own research.