That's a great question.
Usually messages that inspire me are those that contain what I feel are messages of the truth and messiness of the reality of the human condition. Those tend to be fiction books for me, oddly enough. Some of the ones that have taught me the most are
1. All The King's Men-Robert Penn Warren
2. The Awakening-Kate Chopin
3. The End of the Affair-Graham Greene
4. The Keepers of the House-Shirley Grau
5. Mystic River-Dennis Lehane
6. The Brothers Karamazov-Dostoevsky
7. The Allegory of the Cave-Plato
8. Revolutionary Road-Richard Yates
9. 1984-George Orwell.
10. Notes on a Scandal-Zoe Heller.
The book I'm currently reading, White Oleander by Janet Fitch, could turn out to be one of the tops on that by the time I'm done with it. All of the books I've mentioned above have deeply flawed, conflicted, and damaged characters that through their words and actions tell stories about how life really is or how it could be. iThese books tell stories of good and evil, salvation and damnation, so to speak. They've all encouraged me to look at the world from other points of view.
As for non-fiction, I've found books like "The Audacity of Hope" by Barack Obama, "Giving" by Bill Clinton "Bono: In Conversation" and " Night" by Elie Wiesel have had a big impact on my life. The first 3 have all fostered a desire in me to take my beliefs and convictions and do what I can to begin to act on them and eventually make the world better. "Night" just opened me up to the disasters of what hatred, rurcism, corruption and turning a blind eye to the problems in this world can lead to. We read that book in my 8th grade languag arts class, and it's the only one that stuck with me. The hope it gives even in the middle of the darkest situation imaginable has had a huge impact on me.
There are others, but I would say these are the main books that stick with me and inspire me to look at life and reality in a new way.