Daniel, I fully share your opinion but pragmatically speaking, Michael is right !
However, as long as a summary is needed, the work of Adm. Spindler (Der Handelskrieg mit U-Booten) is already a great resource and the first 3 volumes of 5 have been translated into english. Seemingly, the interest of a few dozens of researchers, compared to thousands of those interested in WW2 is not enough for having the last 2 books translated. Fortunately, I know enough of german to deal with that nearly alone but I understand your feelings. Some years ago in UK this translation had been intended but at evidence, the project failed and here we are today.
Best regards
Yves