Where to publish comments on a scholarly book?
From your description, about the most it sounds like you could do is post your notes online.  

Why?  Your notes are not self-contained and I haven't seen things like "study guides" published in concordance with graduate texts, which indicates that there's little market.  Even if there were some market for this, if you don't have time to make your notes self contained, you likely don't have time to get them in a publishable format either.  In any case, the amount of money you would get should be minimal.  Grad texts are not a cash cow.  I also don't see much advantage in an ISBN, but you could get an arXiv id if you post to the arXiv.

Free online notes can give you just as much if not more "recognition" than publishing, depending on the situation (free notes are easily accessible, hence can get wider distribution).  I've done this for some courses I've taught, and while I don't have any measure of how much recognition I'm getting (nor is it that important), sometimes I meet people at conferences or what not and they say nice things about my notes.  Conversely, I've gotten a good impression of many young people by coming across some nice notes they've written.  

Of course, if you do this, you should make sure to give the author due credit and that you will not be infringing on copyrights.  Note that the presentation of material and choice of definitions is a big part of the originality of the author's manuscript.  I spend a lot of time thinking about order of presentation when I write notes.