You may be interested in /u/restricteddata's answer to the question How close was Heisenberg to successfully developing the Atomic bomb for the Nazis?. There are a couple more answers on this subject by him in our FAQ. Also he recently made a post on his blog titled When did the Allies know there wasn’t a German bomb?

Karlsch's work has been mostly been ignored by historians, because there are severe technical and historical problems with his interpretations and arguments. He has found some interesting documents but I think he has not interpreted them correctly. Not a shred of compelling physical evidence has been found that the Germans made any significant progress, and the only textual evidence that makes one raise an eyebrow consists of rumors circulating in Soviet intelligence circles at the time. It is a weak foundation to base a big claim.

I have written a bit more about some of the larger methodological issues here — there are some big hurdles to overcome if one wants these kinds of claims to have the benefit of the doubt (namely, that one cannot just make a nuclear weapon in one's basement, especially in the 1940s — there ought to be copious evidence if such a thing occurred, and there is none). I have a footnote on Karlsch's work on the post, as well, which talks about some of the technical issues involved (e.g. pure-fusion bombs of the sort he sometimes claims they made don't seem to actually be feasible).