Read mediabistro.com's GalleyCat. GalleyCat is a blog offering information on book publishing, including a special section on deals. Articles here include what literary agents sold which works to whom and for how much. You can also subscribe to GalleyCat and receive book deal news as it happens.
Subscribe to Publisher's Lunch. Publisher's Lunch is the book industry's daily essential read. Members of this list can read daily a report of recent book deals, as well as other publishing insights. There is a free version with about 25 percent of the news included in the paid deluxe version.
Join Publisher's Marketplace. As of December 2010, the monthly rate is $20.00. Membership includes the deluxe Publisher's Lunch newsletter, as well as access to a deal database containing thousands of agent sales.
Read the Writer's Market. The Writer's Market is an annual print book offering information on over 500 literary agents and publishing houses. Each entry contains recent sales, as well as other pertinent information. The corresponding website, accessible with purchase of the book, offers more recently-updated information.
Follow literary agents on Twitter. Twitter has a vibrant community of literary agents who freely offer advice to aspiring writers, as well as industry chatter among other agents and editors. Use sites like WeFollow.com and Twellow.com to search for literary agents.
Visit literary agent websites. Find names of agents that interest you --- online, in the Writer's Market or in the acknowledgment pages of your favorite books --- and visit their official websites. The news page and agent bio is likely to contain recent sales. The same goes for an agent blog or personal website.
Research agent directories. Online directories, such as AgentQuery.com offer updated directories of literary agents, including profiles that offer deals, clients and other information.