I bought "die sonaranlagen der deutschen unterseeboote" by Eberhard Rössler.
It has quite a bit detailed information on all types
of WWII sonar, and a bit before and after as well.
with unique photo's of some boats (primarily
e.g. anybody ever heard about the Onion-bow ?
in the end some new boats got a sonar bulb on the
upper bow, very much like the 50/60-er russian
boats. U889 was test boat for it and there are
three pictures of this boat and it's unique bow
And what about the Fafnir anti-sonar trick ?
apparently they put pyramid shaped cones on
U11 in 1940 (!) to lower it's sonar signature.
later they put a rubber layer on some boats
(alberich, they called it).