@kbaerwald:
The book you attached was in some very weird format, probably some old (or extremely new?) .mobi version. I simply imported it into calibre, let it convert (yes, input format=mobi, output format=mobi), and verified that the language was correctly set. That produced a file which is about 1/3 of the size of the file you posted, and which is hyphenated properly. I'm attaching the working one.
You may want to give this method a try.
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