John Xántus was a bit of a charlatan; of that there is little doubt. He lied about his exploits, joined the U.S. Army under an assumed name, and managed to alienate most of the people he met. Yet this Hungarian immigrant became one of the Smithsonian Institution's most successful collectors of natural history specimens in the mid-nineteenth century, and he is credited with the discovery of many new species in the American West.   From his station at Ft. Tejon in California's Tehachapi Mountains, Xántus carried on a lengthy correspondence with Spencer Baird at the Smithsonian, to whom he shipped the specimens he had trapped or shot in the surrounding sierra and deserts. A prolific letter writer, Xántus faithfully reported his findings as he bemoaned his circumstances and worried about his future.   Working from Smithsonian archives, natural history writer Ann Zwinger has assembled Xántus's unpublished letters into a book that documents his trials and triumphs in the field and reveals much about his dubious character. The letters also bring to life a time and place on the western frontier from which Xántus was able to observe a broad panorama of American history in the making.   Zwinger's lively introduction sets the stage for Xántus's correspondence and examines the apparent contradictions between the man's personal and professional lives. Her detailed notes to the letters further clarify his discoveries and shed additional light on his checkered career.