by - Ekman, Per-Olof
(Sea wolves: Submarines and submarine warfare in the Baltic Sea)
Ubåtar och ubåtskrig i Östersjön
1983, Schildt, Helsinki
348 pages, 40 photos, maps
A great book which covers both WWI and WWII and all the movements of Russian, Finnish and German submarines in the Baltic Sea. It also covers the operations by British submarines in the Baltic in WWI.