I bought this book, and a couple of others, when I wanted to gain admittance to business school. The business school admission test, the GMAT, has a pretty serious grammar section and it quickly become clear that my English courses from college were simply inadequate. This book did a great job of showing me the real rules of grammar.
You will have to study the book carefully, and work hard to actually improve the way you write and speak, but the effort is worth the reward. I can honestly say that all of my years of public education (top college, solid engineering graduate program, etc.) did little to prepare me for real world writing, but this book helped immensely.
Grammar, in my opinion, is a subtle social indicator. If your school system didn't teach you how to speak well, then you'll find a whole secret society the Grammar-Educated that gently looks down upon the Grammar-Ignorant. Dr. Grammar's book is good, but the important point is to get a couple of grammar books and simply try to learn what they teach.
Good luck.
(Sorry for the preachy review.)
JJ
PS - I scored well (enough) on the GMAT.
PPS - Please don't comment on any grammar mistakes in this review :-)