Short answers, pulled from the story.
Theodore Baker, PhD (1851-1934) originally compiled and published the dictionary in 1900 through G. Schirmer, Inc. Before producing the dictionary, Baker had edited three editions of A Dictionary of Musical Terms for the same publisher, in 1895, 1896, and 1897.
Nine editions have been published, from the first in 1900 to the ninth in 2001. The ninth edition, billed the Centennial Edition, was the most recent and expanded the work to six volumes.
Nicolas Slonimsky (1894-1995) served as editor from the fifth edition in 1958 through the eighth edition in 1992. He expanded the fifth edition to 1,855 pages and rigorously verified biographical details including birth and death dates using archival sources.
The ninth edition, published in 2001 and called the Centennial Edition, was the first to span multiple volumes, filling six volumes total. It was edited by Laura Diane Kuhn and focused on expanding coverage of popular music and jazz.
Schirmer Reference, the imprint that holds Baker's, is currently owned by Gale, a division of Cengage Learning. G. Schirmer was originally sold to Macmillan Inc. in 1969, and the reference holdings were retained when the music publishing side was sold to Music Sales Corporation of London in 1986.
Notable earlier works include John Weeks Moore's Complete Encyclopedia of Music published in Boston in 1844, and George Grove's A Dictionary of Music and Musicians published in London in 1879. A contemporaneous American work was Oscar Lee Thompson's International Cyclopedia of Music and Musicians, published in 1939 by Dodd, Mead and Company.