Here's another Inkmesh link for free submarine ebooks (using the subject search)
http://inkmesh.com/search/?qs=subjec...nd+Free+Ebooks
Free Navy books:
http://inkmesh.com/search/?qs=subjec...nd+Free+Ebooks
A couple of other u-boat ebooks:
Diary of a U-Boat Commander
U-Boat Hunters
And here's
The Hunt for Red October, not free though.
Hope this helps.