Just posted the following in the Rolling in Bf 110 topic, and then remembered that this topic was about manuals too.
I found all kinds of pilot manuals for the Bf 110 series (and Me-210's, Me-410's, Bf-109's, Fw-190's, Ju 87's, Ju 88's, Ju-188's, He-111's, He-162's, Me-262's, Do-17's), from early to late models. Even several different ones for various G-2 models depending on loadout options. Including the Mk 108 version.. It even has illustrations/charts showing the ballistic path of the 108 shells compared to the 151/20 shells. It's all in German.
I found the manuals here:
http://deutscheluftwaffe.de/archiv/D.../Dokumente.htm - Click on "F" and then
scroll down to "Flugzeug Handbücher". Then on the manufacturer.
EDIT: Crappy, been looking through the 110 ones, and they are mainly just technical manuals, not really for piloting. Not the gold mine I thought it was.