Here's a fascinating case-study on how Timothy Ferriss, author of The 4-Hour Workweek: Escape 9-5, Live Anywhere, and Join the New Rich, prototyped and tested various titles and covers for his book, by buying various Google AdWords (for the title), and sneaking dust-jackets onto various business-books at the local Palo Alto Borders and watching how people reacted to them (for the cover).
He took 6 prospective titles that everyone could live with: including 'Broadband and White Sand', 'Millionaire Chameleon' and 'The 4-Hour Workweek' and developed an Google Adwords campaign for each. He bid on keywords related to the book's content including '401k' and 'language learning': when those keywords formed part of someone's search on Google the prospective title popped up as a headline and the advertisement text would be the subtitle. Ferriss was interested to see which of the sponsored links would be clicked on most, knowing that he needed his title to compete with over 200,000 books published in the US each year. At the end of the week, for less than $200 he knew that "The 4-Hour Workweek" had the best click-through rate by far and he went with that title.
His experimentation didn't stop there, he decided to test various covers by printing them on high quality paper and placing them on existing similar sized books in the new non-fiction rack at Borders, Palo Alto. He sat with a coffee and observed, learning which cover really was most appealing.