You can forget Harbrace College Handbook or Strunk and White Elements of Style as your definitive writing reference guides. The best book I have run across, and the book I most often use when writing or grammar questions arise is the Writer Source 2000. This is a junior high school guide to writing, thinking and learning; and the most understandable, convenient and definitive book on writing in my library. It is available used through Amazon for as little as four dollars.
If I only had one writing guide, this would be my pick.