This book is boring and doesn't offer any new insights into SP's life. The author has a nasty habit of comparing Plath to other women (Marilyn Monroe, Susan Sontag) in ways that seem random until you realize he wrote biographies on them, too. Knowing that SP shared superficial similarities to two women I'm not interested in reading about really does nothing to broaden my understanding of her.
Things finally pick up in the last chapter, which covers the Hughes family's (mis)handling of Plath's estate, and I also enjoyed the appendices. I was shocked to see that the author interviewed people who knew Plath since the overview of her life is so dry. If you've read her journals or another biography then you really don't need to sit down with this book.