Upon researching an online source for a regimental history I stumbled over this site. They offer quite a lot of books on uniforms, regiments ets. as PDF. Regretably they are not cheap but might be of some help if you are really desparate:
http://www.historischer-bilderdienst.de/ (click on "Uniformenkunde" and then select one of the states)
Some previews are detailed http://www.historischer-bilderdienst.de/images/kontaktkopieu128.jpg, http://www.historischer-bilderdienst.de/images/kontaktabzugu031.jpg
Here is a catalog for a quick search: http://www.historischer-bilderdienst.de/downloads/katalog.pdf
I ended up finding my regimental history on the following site, which might be already well know but at least it was new to me; he takes €5 for each scanned book: http://www.military-books.de.vu/
http://www.historischer-bilderdienst.de/ (click on "Uniformenkunde" and then select one of the states)
Some previews are detailed http://www.historischer-bilderdienst.de/images/kontaktkopieu128.jpg, http://www.historischer-bilderdienst.de/images/kontaktabzugu031.jpg
Here is a catalog for a quick search: http://www.historischer-bilderdienst.de/downloads/katalog.pdf
I ended up finding my regimental history on the following site, which might be already well know but at least it was new to me; he takes €5 for each scanned book: http://www.military-books.de.vu/