For students of the Old West, Josiah Gregg's 1844 Commerce of the Prairies is the preeminent source, still, on the development of the Santa Fe Trail; but until the publication of his disinterred diaries and letters, in the early 1940s, Gregg himself was a shadowy figure, and he remains among the least known of--in Paul Horgan's felicitous phrase--our ""national autobiographers."" On that score alone, this small volume, containing Horgan's introductions to the two volumes of personalia (now o.p.) and an interpretive essay (from Southwest Review), would be welcome. But Gregg's life and personality also have an intriguing mythic aspect. He was apparently a neurasthenic who regained his health in the wilds, a cultivated man contemptuous of his fellows--one of those gifted misfits, in short, for whom the West provided an escape and a point of re-entry. The scenario is operatic: Gregg rallying visibly each day of his first, rugged journey to Santa Fe and, years later en route to California, so antagonizing his bone-weary companions with his incessant note-taking and measuring that they are ready to drown him--in the stream thereafter called the Mad River. As an observer, Horgan demonstrates, Gregg was attentive to ""curious and unexpected details,"" and markedly respectful toward the Indians. (One, shown the rapid fire of a repeating pistol, ""seemed""--in Gregg's words--""to grasp the secret instantly, and, drawing his bow once more, he discharged a number of arrows with the same rapidity, as a palpable intimation that he could shoot as fast with his instrument as we could with our patent fire arms."") These pieces, however, are as much Horgan as Gregg--which is to say exquisitely tooled, and elegiac--and they were clearly meant to complement the texts. But Commerce of the Prairies, at least, is available in several editions and historians--most recently, Frederick Merk--continue to doff their hats invitingly to the frontiersman/scholar Gregg.