Find your author's website, if he has one. Most authors do and will have a place to send an email from the site.
Find the name of the author's publisher inside the front cover of the book. Write directly to the publishing house and ask to have your letter forwarded to the author.
Check the publishing house website for information on authors and how they can be contacted.
Research the name of the author's editor or agent. Editor's in different houses take on projects and work directly with the author. You might find the agent's name in the Acknowledgments.
Search Internet resources like Literary Marketplace to get the latest information about authors, what they are writing and where they can be contacted. This site requires a subscription.