Copy/Paste from Book to Book

Seems I miss something in the documentation of FM7.1.

I want to use the clipboard to copy the content from an opened book into another book (basically to temporarily migrate from one EDD/template to another).

But F_ApiCopy(file, FF_INTERACTIVE) returns "FE_BadDocId" consistently. I use

     "er.beg.childId  = F_ApiGetId(FV_SessionId, file, FP_HighestLevelElement);

      F_ApiSetElementRange(0, file, FP_ElementSelection, &er);"

to select the root element.

For documents (i.e. .fm, not .book and using

     "er.beg.childId  = F_ApiGetId(file, F_ApiGetId(FV_SessionId, file, FP_MainFlowInDoc), FP_HighestLevelElement);

     F_ApiSetElementRange(0, file, FP_ElementSelection, &er);"

) this concept works perfectly, so there must be a difference!

Thanks in advance for any pointers.