Hi Simon,
$8 is a bargain for Enemy Sub, it is one of my favourite books. Wolfgang Frank was board U 47 for the boat's 9th patrol (just like Buchheim was on U 96). I'm halfway through writing a patrol history of U 47 using the KTBs - Franks' book has been very useful.
Snyder's book sheds light on a lot of the controversies that were brought about by Korganoff's "Phantom of Scapa Flow". Another excellent book is "Nightmare at Scapa Flow" by HJ Weaver, which cleared up the controversies very well.
Enjoy both books,
Dougie