TO THE FRONTIER by Geoffrey Moorhouse (Holt, Rinehart, Winston: $17.95). Adventurous or armchair travelers will enjoy this detailed report of a three-month journey through the huge, troubled area between India and Afghanistan. The writer, unflappable optimism matched only by his boundless curiosity, manages to wring every possible advantage from his opportunities. He carried only a backpack and traveled mainly by crowded train or ancient vehicle across the often dangerous and sometimes desolate countryside. His travels took him from Karachi to the northwest Frontier, across the Baluchistan desert and east to Lahore, north to Islamabad and Peshawar, then on to the Khyber Pass to the Afghan border, skirting the China border and then hiking through the mountains to Gilgit. Moorhouse's account of how he managed to talk his way through the Khyber Pass, even though it was sealed shut (in a vain attempt to stop drug smuggling), will impress those who are more likely than he to take "no" for an answer. He includes an impressive amount of history in his account, although his everyday observations are the highlights throughout.