There is a select group of performers and conductors inextribly linked with specific composers. Schnabel and Beethoven, Casals and Bach, Toscanini and Verdi; the list goes on.

It's time to reserve space for Michael Tilson Thomas and George Gershwin, allowing space for an asterisk: Thomas qualifies as both a performer and a conductor.

This versatile musician is well on his way to becoming the definitive double-threat Gershwinite. He's a natural for the role. His father studied piano with Gershwin; his uncles performed and wrote music with him. And now there's his latest recording, simply titled "Rhapsodies" (CBS MK 39699 Digital), a refurbished "best of" and "unheard of" collection that finds Thomas on both sides of the baton.

He's restored the original orchestrations of Gershwin's two rhapsodies -- the famous one "In Blue" and "No. 2" (also for piano and orchestra). But far more significant are the six newly "found" works (counting this version of "Rhapsody No. 2") taken from unpublished melodies, a piano sketch and manuscripts hitherto unobtainable or feared lost. He's dedicated the album to Ira Gershwin, George's brother, who tirelessly served as adviser and liaison right up to his death in 1983.

Thomas is no stranger to historic "firsts." Thanks to the miracles of studio technology, he actually recorded "Rhapsody in Blue" with Gershwin 10 years ago using the jazz band format first conceived by Ferde Grofe' in 1924. Thomas conducted the Columbia Records Jazz Band, and Gershwin appeared courtesy of a pair of Duo-Art piano rolls he made in 1925. The results were mixed. Gershwin's performance, exciting and puckish in spots, revealed how heavy-handed the piano part has become in some interpreters' hands. But the coordination of band and piano parts, made a half century apart, fell victim to the necromancer's art. Grossly accelerated tempos so trivialized this "Rhapsody," it sounded like cartoon music.

That Thomas-Gershwin collaboration was a shade more than two minutes shorter than the present Thomas-Thomas reading. As a conductor, he keeps tight control of the tempos without dampening the expressive power of the Los Angeles Philharmonic. The trademark opening clarinet slide is indulgently sassy, in a streetwise fashion; and the saxophones have an immediacy missing on other versions not recorded digitally and with a small group. Thomas the pianist stacks up favorably with Gershwin; his touch is elegant, though he's given to capricious mood changes.

The effectiveness of Thomas' conception can be measured in more up-to-date terms by listening to Andre' Previn and the Pittsburgh Symphony Orchestra's rendition (Philips 412 611-2 Digital; -1 LP; -4 cassette). The full orchestra treatment seems brutish by comparison, devoid of charm. Previn, who like Thomas does double duty as conductor-soloist, has strong jazz credentials, and there's much to admire in his evenhanded execution. It's wasted because he's fighting an uphill, and ultimately losing, battle. "An American in Paris" and the "Piano Concerto in F" fill out this all-Gershwin disc.

Thomas best makes his presence felt in the first-time recordings, which impress with their soft-spoken lyrical refinement. "Promenade" ("Walking the Dog") was part of the 1936 film "Shall We Dance?," starring Fred Astaire and Ginger Rogers. Written for small orchestra and two pianos, it's the antithesis of a typical full-blown Hollywood production number. Thomas views "Promenade" accordingly, investing the arrangement (which surfaced at the RKO Studios warehouse in 1978) with a carefree, delicate lilt.

The piano selections offer an intimate glance of Gershwin behind the scenes. "For Lily Pons," named by Ira, who knew his brother was planning to write music for the French soprano, is a gentle fragment that ends scarcely after it starts. Ira might well have given "Sleepless Nights" its title, since he labored unsuccessfully to come up with a suitable lyric for this captivating, yet unpublished melody. He had better luck naming "Violin Piece" (after George's appreciation for the instrument), and supervising Thomas' transformation of a mere 32 bars into a splendid song without words.

Thomas and Gershwin are a perfect match. A much more unlikely pair are Thomas and Charles Ives, the iconoclastic New Englander concerned with writing music to please him, not the uncaring public or the greedy publishers. Chalk one up to his versatility that Thomas directs vivid, clearheaded accounts of two Ives orchestral works, "Symphony No. 3" and "Orchestral Set No. 2" (CBS MK 37823 Digital), which here make their recording debuts in the latest critical editions prepared by the Charles Ives Society.

The mediator between composer and conductor is the Concertgebouw Orchestra of Amsterdam, one of the world's foremost ensembles, but one whose heart is closer to The Hague than the Housatonic. They're ripe for the challenge, and in these recordings made at the Concertgebouw, they add clarity to Ives' prickliest sections by rounding off the sharp edges, like a sonic pumice stone. Their impact is twofold: They make Ives sound good and they make Thomas look good.

The Third Symphony, the more conservative of the two pieces, won Ives a Pulitzer Prize in 1947, 40-odd years after the fact. The three movements liberally quote popular hymn tunes to depict a rustic Connecticut revival meeting. Ives' contrapuntal invention, dissonances and genial reweaving of melodic threads wear well. The unflappable Concertgebouw is right at home. For 23 minutes or so, they're Yankee compatriots.

A more demanding work is the "Orchestral Set No. 2" (as yet unpublished), which dons its patchwork cloak of Americana as envisioned by Ives -- folk songs, dance band snippets and hymns -- with crusty defiance. Ives, who shared Emerson's belief that "all things are one," reconciles all elements. Complex tonal and rhythmic schemes assault, cajole and insinuate as they intertwine, until the climactic final movement in which the brass dish out a fanfare of the hymn tune "In the Sweet Bye and Bye." Thomas and the Concertgebouw Orchestra clarify rather than muddle the jumble of ideas, and the CD's warm, but not excessively bright, sound is ideal.

Ives once observed, "If you want something played, write something you do not want played." Perhaps Thomas and the Concertgebouw Orchestra will record more works Ives wouldn't have wanted to hear. Wouldn't put it past them.