Very awkward, not because for whatever title/s you choose you will find detracting reviews, but because there are a number of specialised areas which have their own respective studies. However, for a better than skeleton overview with authors conclusion, (valid but not gospel), I would suggest 'Hitler's Spies: German Military Intelligence in World War II' by David Kahn
My interests in the core operation I found centred on Canaris, and from this perspective I would suggest Canaris by Heinz Hohne (English translation).
The why did he do it question over what did he do, established another gravy train for writers to climb aboard with impunity. Fortunately for them, and unfortunately for simple historians, Canaris and most of his confederates and adversaries did not survive the war or Allied retribution. Very little remains in respect of personal papers or dossiers, (at least whats generally known of or disclosed), defining his personal agenda and any who were party to it, hence established facts are continuously used to add validity to an authors interpretation.
Bassetts book does not appear to have escaped this line of criticism levied at the authors' conclusions, though one reviewer makes the case that claims of a successful German network in Britain are absurd. If this is a feature of the book, I think the world and his wife would agree with very good reason.
Release of previously classified material usually spurs some new works, however, in this case I dont believe anything major has come to light since the OSS releases of 1975? New interviews are welcomed, but if these are recent then surely one can expect that the people have already made statements in the past, and there are considerably fewer to approach as most are now dead?
I now there are books centering on the theme of whether or not Canaris was a British Agent, or where Heydrich was assassinated to stop him pursuing a case against Canaris, but I tend to agree with those who see these as nothing more than angles to justify a new version of what has already been covered by others in context.
Certainly a very interesting read, complete with interviews, declassifieds, documented events and reasonable (though not unopposed) conclusions, would be the combined (I & II vol.) work Bodyguard of Lies by Anthony Cave-Brown. Researched and written over some 14 years with revision for new releases, this 900+ page book attempts to deal with most angles. While Cave-Brown enters areas which he cannot be expected to have examined to the nth degree (and therefore where it is most susceptible to error), it provides a very good platform from which the reader can chose areas to investigate further for themselves.
My understanding is there were two world altering opportunities missed, with the very best intentions.
In general, Canaris was sure Hitler would be dissuaded from attacking Czechoslovakia if he could be convinced an attack would invoke military reprisal from Britain and France. What he sought from Chamberlain was a formal degree of aggressive posturing. Failing this, or coupled with this, the German military would seize Hitler and his hierarchy and impose Marshall law.
The British declined in the belief such was more likely to provoke German aggression, and, Hitler was only posturing himself, and, if it came to it, the German military would not revolt. A peaceful solution should be sought.
The conspirators saw the only course was therefore a coup when Hitler implemented an attack, and everything was poised for this. Then, days before all was to happen, Chamberlain turns up with a virtual whatever you want deal for Hitler, and the trigger and reason which was to validate the coup to the German people is removed.