Vladimir Horowitz was working on an album when he died last Nov. 5 at the age of 86. This week The Last Recording is being released by Sony Classical with a closing selection, ”Liebestod,” that has an eerie resonance. The piece ? ”Love-Death” in English ? is a Liszt piano transcription of an aria from Wagner’s Tristan und Isolde , in which Isolde bids farewell to her dead lover, Tristan. Most of the version of this piece heard on the new album was played during Horowitz’s final recording session on Nov. 1.

The Last Recording caps the brilliant career of the man considered by many to be the greatest pianist of the 20th century. It also marks the end of a remarkable collaboration between Horowitz and his longtime record producer, Thomas Frost. In a career that has included work with such conductors as Leonard Bernstein and Bruno Walter and such pianists as Glenn Gould and Rudolf Serkin, Frost considers his association with Horowitz ”one of the highlights of my musical life.” Even now, Frost has trouble accepting Horowitz’s death. ”His spirit,” Frost says, ”is all around me.”

Very few people worked more closely with Horowitz than Frost. The two first collaborated in the 1960s, after the pianist switched to the CBS label, where Frost was a staff producer. This affiliation ended in 1973, after more than a dozen records together, when Horowitz left CBS; it resumed in 1985 when the pianist moved to Deutsche Grammophon, where Frost, as a consultant, produced five Horowitz albums. Eighteen of the 23 Grammy awards won by Horowitz were for albums produced or coproduced by Frost, including Horowitz’s two famous ”comeback” concerts, Carnegie Hall in 1965 and Moscow in 1986.

Frost first met Horowitz in 1962, but he had long been a fan. While a music student at Yale in the late ’40s, Frost managed to procure a dozen or so photos of Horowitz at the keyboard to study the master’s technique: ”I wanted to see how he was sitting and how his fingers were.” Horowitz impressed Frost with his tremendous range, from ”the emotional thrust and those thunderous sonorities” to ”the delicate classical simplicity with which he could play the pianissimos.” And what was the secret of the great pianist’s art? ”The mind,” Frost says. ”He has a concept of what the music should sound like before he plays it, a degree of imagery that is not known to the average person.”

While recording, Horowitz almost always played entire pieces or, if extended works, complete movements. Even when he needed to touch up short passages because of misplayed notes, Horowitz would start at the beginning and run through to the end. ”A session with him was like a little concert,” Frost recalls. For The Last Recording , the immediate audience was only two: Horowitz’s wife, Wanda, and a piano technician. As with his 1989 recording Horowitz at Home, the pianist played in the living room of his Upper East Side Manhattan town house. Foam padding was deployed around the room to reduce echoing in the space. To monitor the sound of the recording, Frost sat in an anteroom off the living room and his engineer sat in the basement.

In the ’80s, Horowitz left the editing of session tapes to Frost. In order to produce ”clean” versions of pieces ? without wrong notes ? Frost would choose passages from different takes. It wasn’t easy. ”The dilemma,” Frost says, ”was that you end up with complete performances that are not different in terms of overall conception, but often are very different in small details.” While the varied tempos, phrasings, and volumes gave Horowitz’s work freshness and spontaneity, they also made it difficult to construct seamless final versions.

The Last Recording will not be Frost’s last Horowitz project. A wealth of unreleased CBS material sits in the vaults now owned by Sony. Frost produced one album from this material last year, and several more will follow. Working with Horowitz, he says, ”has been a great adventure.” And it’s not over yet.