David Irving also started a book project on this theme 'Secretly Overheard' in 1988:
http://www.fpp.co.uk/books/Overheard/
This project never saw print for various reasons which Irving notes in his post. A reconstruction of his manuscript can be downloaded from here:
http://www.fpp.co.uk/books/Overheard/CSDIC.pdf
I am posting this purely for those who might find it helpful. It is not my intention to start a debate about Irving's credentials as a historian.