@ slex: I have the same Problem! I read, that the main problem is the old version of htmldoc (I think a converter) which is used by web2book. Htmldoc won't be updated anymore and doesn't support the UTF-8 characters, which is used by many newer websites. web2book supports iso-8859-15, which includes german umlaute, but I don't know, if there is a way to transform UTF-8 in iso.
If anybody has help, very welcome!!