I suffer from it all the time. "Oh God, I don't know ALL these rules. Here we go, I'd better do another Google on 'compare to versus compare with', or on my favourite 'which vs. that' agghhhh.
Polishing people's work brings with it an unspoken commitment that you can spell, you know what subject-verb agreement is, but without these books I'd be doing a lot more googling:
1) The Oxford Dictionary for Writers and Editors - indispensable guide to all things. What's the proper abbreviation for Euro? When does good quality have a hyphen? Spam or spam?
2) The Penguin Guide to Punctuation - gapping commas, semicolons, question marks; it's all there, but what I really like about this book is that it's readable and has plenty of practical examples.