A: Fantastic. Plot, characters, themes, writing--all worked for me. The book stayed with me, and might very well end up on my keeper shelves. Ex: The Grand Sophy, Georgette Heyer.
B: Pretty darn good. I might have some problems with the book, but it make me think, was over all well done, and is something I'd recommend. Ex: Spymaster's Lady, Joanna Bourne
C: Neither good nor bad. It's fine, but not something I'd recommend, nor something I'll reread, probably. Overall, the book might work, but not really for me, and I might have some big issues with it. Ex: Again the Magic, Lisa Kleypas
D: Not great. Probably leaves me thinking, why did I buy/read this. Has some massive problems, in theme, characterization, plot, writing. Ex: Dark Prince, Christine Feehan
F: Terrible. How did this get published? There's probably a massive fail in it. It might be something I'll pass on, but not in a good way. Ex: (nothing comes to mind at the moment, probably since I try and forget them as soon as possible, as soon as I remember one, I'll stick it in)
DNF: Pretty self-explanatory. Did not finish. Either because it's all around so bad, or it hits an absolute no-no for me.