The search giant is being sued by publishers and the Authors' Guild over the digitisation of thousands of books which they say was conducted without authors' and publishers' consent. Google is pursuing a plan to digitise the contents of four US university libraries, the Oxford University library and the New York Public Library.
It had issued subpoenas to Yahoo! and Amazon.com seeking details of the book digitising programmes of those two companies. Google sought commercially sensitive details from its competitors, such as costs, details of discussions with publishers and sales estimates.
Yahoo! has now joined Amazon.com in rejecting Google's request for information.
The request was an attempt to gain access to trade secrets, Yahoo! suggested. "There is simply no need for Google to be peering into the minds and computers of Yahoo employees," wrote Yahoo!'s lawyer in a list of objections to the request it sent to Google last week.
Authors' representatives and publishers sued Google in a federal court because it proceeded with its programme without their permission. Microsoft and Yahoo! are both involved in book scanning activity but with the co-operation of the book industry in a programme called the Open Content Alliance.
The OCA has as a specific aim, the principle that all scanned books will always remain in the public domain, and has the backing of major libraries including the UK's National Archives.
Google had said that the court would protect any commercially sensitive information sent by competitors to it, but the companies have still rejected the request.