Yes the cover sucks, it sucks horribly, however, the book is by far the best book I have found for people new or intermediate to design.
This seems like a dichotomy but its not. The reason the cover sucks is that Williams is not naturally good at design. This means for her to get good at it she had to actually learn and understand it. This is why this book is so good. However, she never learned how to design with color. Most of the designs she makes using color I feel are bad.
That said this book ROCKS, is hands down the best book when it comes to design basics of:
Proximity
That's the publisher's house style, I'm pretty sure. Author's rarely have control over book covers or book titles--publishers think they know more about such marketing issues than authors do.