The Grove Book of Opera Singers

It's a dictionary of singers and roles in operas. The 640 page book by Laura Macy was published at Oxford University Press, USA and released on October 2nd 2008. Macy’s new volume offers biographical and professional information about more than 1,500 opera singers active from the sixteenth through the twenty-first centuries. She explains in her preface that many of the entries are drawn from The New Grove Dictionary of Opera (1992) and The New Grove Dictionary of Music and Musicians (2d ed., 2001). Each brief article includes birthplace and date, education details, date of professional debut, and important subsequent appearances at major opera houses.

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Both Angela Gheorghiu and Roberto Alagna are listed here. In fact, the page about Roberto can be viewed on Amazon.com

If you're interested in the topic, there are several books published under the title of Grove Books:
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Grove Books of Operas - published on April 1st 2009
The New Grove Guide to Mozart and His Operas - published on Februari 27th 1997
The New Grove Guide to Wagner and His Operas - published on August 15th 2006
The New Grove Guide to Verdi and His Operas - published on February 26th 2007