Mitt Romney announced Thursday that he's running for the Republican nomination for president. The entry has been long expected from the former Massachusetts governor. Several high-profile Republicans -- including Newt Gingrich and Ron Paul -- have already hit the campaign trail. Others who are less well-known, like former Minnesota Gov. Tim Pawlenty, have also gotten an early start.

And then there's Sarah Palin, who is traveling by bus to campaign-ready states -- she's attending a New Hampshire clambake Thursday -- though she has not yet announced her intention to run.

Can the popularity of the candidates' books signal how well they'll do with the American public? Jacket Copy looked at how the hardcover editions of their books were faring at Amazon.com at midday Thursday, after Romney's announcement. Despite his news, Romney's book falls short. It's Ron Paul who has captured readers' attention -- his "Liberty Defined" has the highest ranking of all the candidates' books at No. 337 overall on Amazon.

If current book sales rankings were votes, Paul would lead, with Palin second and Romney third. Here's a comparison of GOP contenders as authors:

1. Ron Paul, "Liberty Defined: 50 Essential Issues That Affect Our Freedom" published April 2011. Amazon rank: 337 2. Sarah Palin, "America By Heart: Reflections on Faith, Family and Flag" published Nov. 2010. Amazon rank: 9,193 3. Mitt Romney, "No Apology: Believe in America," published Feb. 2011. Amazon rank 16,639 4. Herman Cain, "They Think You're Stupid: Why Democrats Lost Your Vote and What Republicans Must Do to Keep It" published May 2005. Amazon rank: 45,728 5. Newt Gingrich, "To Save America: Stopping Obama's Secular-Socialist Machine" published May 2010. Amazon rank: 71,027 6. Tim Pawlenty, "Courage to Stand: An American Story" published Jan. 2011. Amazon rank: 73,198

Gary E. Johnson doesn't appear on the list; his website is currently down, but I don't think he's published a book. Let's qualify that with a "yet."

The surprise might be that Gingrich's book lags so far behind Cain's. Gingrich has had more exposure on the campaign trail, is a known author and his book is new; Cain's was published six years ago. Maybe he's a candidate to watch.

-- Carolyn Kellogg

Photos: Mitt Romney, left, announcing his candidacy on Thursday; Ron Paul signs his book "Liberty Defined" at an appearance in May. Credits: Darren McCollester / Getty Images; Ethan Miller/Getty Images