I've always been fascinated by language (my mom, the elementary school librarian, found a book in her to-be-discarded pile about word origins, and brought it home for me - being that I had been the only kid to ever sign it out, 15 or so years before) but this book really tested that fascination.
If you want to wade through a dense thicket of information on the history of spelling, it's great. If you are really trying to improve your spelling and you're looking for ways to do so, skip the first 2/3 and just read the bit on the exercises at the end - if you try to wade through the first part, you'll never make it.
Even the suggestions given for working on spelling would daunt anyone but the most determined. She basically wants people to carry a dictionary around with them at all times, along with a notebook for listing words you have difficulty spelling. If this method works for you, go for it! I heartily applaud you... but I think most people would find this book less than helpful.