In Hamburg I found bookshop Fuchs (rödingsmarkt)
It has a huge collection of boat books.
I bought three books by Lothar-Günther Buchheim
(who also wrote Das Boot)
1) Die U-boot Fahrer
2) U-boot Krieg
3) Zu tode gesiegt
(all in german)
These books are documentaries, not fiction.
As Mr Buchheim was a war correspondent,
these books are full of very large, crisp
pictures of type VII boats.
nice addition to the library !