AutoTrader.com, a company that provided information for consumers buying, selling and researching cars, agreed to acquire Kelley Blue Book, a rival information service best known for its "blue book" valuations of used cars that have become part of the U.S consumer lexicon.

The companies didn't disclose the terms of the transaction, but its importance to prospective car buyers is clear. In some ways it will streamline the research that goes on before many prospective buyers walk into the showroom. However, people shopping for...