Strunk & White's is more or less essential as a reference. I've got it memorized by now, but I used to refer to it constantly.

Another excellent guide -- though less focused on grammar -- is Ursula LeGuin's Steering the Craft. LeGuin's a master, and the book is just brilliant.

Finally, for an honest-to-god grammar reference, try Diana Hacker's Bedford Handbook. We used this in my Advanced Grammar class in college, and I used to swear I'd burn the thing in effigy when I was done the class, but really it's very useful. Clean and concise.

Generally speaking -- did your editor say, or can you tell yourself, where your main problem is? Sentence level, paragraph level, chapter level? A lot of people think that grammar's something unimportant that editors fix (hint: they don't), but the good news is that they just take practice to fix. Problems on a larger level, like exposition issues, bad transitions, clumsiness in description, etc -- are more advanced, but they're also a lot harder to point at and say "yeah, do this there."

(No authority for this, incidentally, besides a few magazine articles -- but I've written two novels and spent a great deal of time critiquing people's work, besides majoring in English, so I wouldn't write me off either. :) )