ok, I found it, it was not the header of a \book .
In the 'master' document in which I include the child-files I have a header file, which is not in a \book , but just a regular \header like for a single score, in which I had a title="Studie" line. That one got repeated as title on the different \bookparts.
I don't know if that is the intended result, because it was not in a \book in the master document. If it is, everything is ok :-). I now deleted the 'title="Studie' from the header of the main file and have it set only in the first \bookpart, which seems to work.
Grtz,
Bart