Having the good fortune to see Wicked when it came through Denver, I was curious as to how it would handle the storyline of Gregory Maguire's novel of the same name. The book, after all, is not uplifting. Most of the characters die, and those who don't become such warped caricatures by the end that Elphaba's death (and the end of the story) finally comes somewhat as a relief.
You'll be happy to learn, then, that, much as Wicked the novel is nothing like The Wonderful Wizard of Oz, Wicked the musical is nothing like Wicked the book. Most of the focus shifts away from Elphaba's various failures throughout her life and instead highlights the now feel-good friendship between her and G(a)linda. The whole story receives a Broadway sheen that (thankfully) makes it virtually unrecognizable as an adaptation of the novel. While some might argue that many of the powerful themes of the book (religious and philosophical questions, social injustice, etc.) were lost, I would reply that they weren't that powerful in the book, anyway, and so their disappearance is no great loss.
In addition, the performances of the actresses were stunning. The most powerful song, "Defying Gravity," induced goosebumps. The sets and costumes were whimsical and imaginative, and the entire production delightful. I would highly recommend you see it if you get the chance.