Thoughts on Wicked: The book and the musical
I see the two as parallel universes. Both of them are amazing and complex, but in different ways.
Musical
Timing, choreography, acting, singing, it being live on stage means you can experience it with sound and sight with many people and their energy. The story of the musical is more optimistic and different in tone. It's intention is to bring the story to everyone.
The Book
The book is darker, a bit more twisted, we get whispered stories, murder, discrimination, propaganda, the seedier side of Oz, we see a girl and her frustrations and the few lights in her life. We get magical words to peruse at our own pace. It's definitely geared towards an older audience, but the ambiguity about what is good and what is evil is still there.
Both
One similarity between the two is Elphaba truly believes in something and is willing to fight for it.
The two mediums were and had to be made differently. Those who would read a book with commentary on governments, discrimination, and a mad Nanny aren't necessarily those who would see a Broadway musical with a funny song like Popular or words like "confusifying". You can enjoy both as long as you recognize there are differences.