has used its lofty stock price to buy three more online companies, including, which runs Web marketplaces for used and rare books and hard-to-find music materials. The price paid for all three was a total of $645 million in stock.

To the book world, the most significant purchase was that of Exchange, which runs, based in Cambridge, Mass. It has a database of more than nine million hard-to-find and rare books, and lists thousands of independent dealers and retailers., Exchange's other site, has more than three million items of hard-to-find recordings and music memorabilia. Exchange, which had previously been known as e-Niche, acquired Bibliofind in January (News, Jan. 25). Amazon CEO Jeff Bezos said these acquisitions would "further increase our selection of rare and obscure items."

Amazon's other purchases were of Alexa Internet, a San Francisco company that makes Web navigation software, and, an e-commerce startup by former Netscape employees that develops technology for online transactions.