Thats funny, I usually am only interested in the 
true NC-17s.  *cough cough* :D 

The Gravitation doujinshi are very explicit and 
are rated R there. I can't imagine anything else 
being worth a higher rating, unless it was some 
sort of extreme fetish, so NC-17 is probably a 
lower rating (maybe they think R is 18+?).. 
especially since the NC-17 samples do look 
tamer, like you said.  How is the sample page of 
the one you want?  The do seem to include 
explicit samples whenever they can, so if your 
sample isn't too "bad", then the rest of the 
book is probably similar.

But thats just my observations from looking 
there just now.. I've never bought from their 
website before.  :)  If you ever get something 
too raunchy, send it my way! ;)  *laughs*