A major shareholder of Borders Group Inc. proposed that the bookseller acquire much bigger rival Barnes & Noble Inc., in a gamble to unite the two giant but struggling retailers at a time of major tumult in the book industry.

The threats posed to the big store chains were underscored Monday, when Google Inc. unveiled its new online bookstore, a retailing venture that adds a major player to a crowd of digital sellers that includes Amazon.com Inc. and Apple Inc.

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