Why, Alex Award, of course!
Named for librarian Margaret Alexander Edwards, the Alex Award celebrates her efforts to introduce teens to adult books. It is given in recognition of books originally published for adults that successfully ‘crossover’ to young adults. Awarded to 10 titles each year since 1998, it is a good place to start looking for titles that deliver.
2004 winners

Read On – More resources to learn about adult titles you might like:
In addition to the Alex Award, booklists and high school recommendations, another good way to learn about adult titles is to read reviews. Three journals published for librarians include reviews of adult books for teens. Also, check out what college students are reading, books reviewed in magazines such as Entertainment Weekly or People, and best-seller lists. Visit your local library for access to these publications.

School Library Journal (SLJ) publishes reviews in a section titled “Adult Books for High School Students.”

Booklist labels reviews of adult books for young adults with the notation: YA/L(limited interest); YA/M(mature); YA/C (curriculum related).

Voice of Youth Advocates (VOYA) publishes 3 useful lists annually: Clueless? (Dec issue) Adult mysteries; Best SF, Fantasy & Horror (April issue) and Best Adult Nonfiction for HS libraries (Oct issue).

The Chronicle of Higher Education publishes “What they are reading on college campuses” featuring a list of bestsellers from bookstores serving over 30 university campuses. Unfortunately, you have to have a subscription to read the list online. The February 25, 2005 issue lists the following top ten titles:

  1. He's Just Not That Into You: The No-Excuses Truth to Understanding Guys by Greg Behrendt and Liz Tuccillo
  2. America (The Book): A Citizen's Guide to Democracy Inaction by the writers of The Daily Show With Jon Stewart
  3. The Broker by John Grisham
  4. The Da Vinci Code by Dan Brown
  5. Love by Toni Morrison
  6. The Five People You Meet in Heaven by Mitch Albom
  7. The Kite Runner by Khaled Hosseini
  8. The Bush Survival Bible by Gene Stone
  9. The Curious Incident of the Dog in the Night-Time by Mark Haddon
  10. Blink by Malcolm Gladwell

Wild Rumpus Bookstore, has a link titled Book Sense Bestsellers that features lists of bestsellers compiled by independent bookstores around the country. For potential crossover books, check out the Trade Paperback Fiction List, the Nonfiction List and their listings for the Remedial Book Club for Immature Adults.