Kaplan owned the autograph manuscript of Mahler's score of his Second Symphony and published a facsimile edition of the score in 1986. Tim Page wrote in "The New York Times": "Only now will musicians, scholars and the general public be able to own a facsimile manuscript of one of the composer's symphonies." On 29 November 2016, the manuscript was sold at auction for £4,546,250, a record for any music manuscript at the time. He also owned the autograph manuscript of Mahler's song, "Ich bin der Welt abhanden gekommen." A facsimile of this manuscript was published by the Kaplan Foundation in 2015. Both manuscripts were, at one time, on deposit at the Morgan Library & Museum in New York City. He was co-editor of the new critical edition of the Second Symphony as part of the Complete Critical Edition of Mahler's works.