There is no question the authors have a deep insight into the case, and clearly they have done a tremendous amount of research - made clear from earlier works such as Frogman Spy.

Is the Crabb Enigma worth reading? 

absolutely, and there is a brilliant photo section and snippets of information that we haven’t seen before. 

'The book is in the Amazon bestseller list for Espionage books'

...this book is hard to put down. 
After all these years, the Crabb story still has the capacity not only to intrigue but to shock.