For those looking for this publication:
Original copies of both volumes are usually quite expensive. Roland's scans here are of very good quality - gleaning more from the book without being well-versed in German may proove difficult.
For those who would like an affordable working copy, there are facsimile reprints available: Literaricon Verlag has both volumes (reprint 2015) and there are at least 2 other publishers also offering volume 2 (reprint 2013 and 2014). I can't vouch for the printing quality though.
I don't want to post any commercial links; if you have problems finding these online, just drop me a line...
I have only found the second volume online (Gayo), scanned from a recent facsimile reprint:
https://books.google.de/books?id=2dpVAQAAQBAJ
Here is a link to the first volume (Batak) which only offers snippets though:
https://books.google.de/books?id=XgNXAAAAMAAJ
Regards,
Kai