TUCKER'S LAST STAND by William F. Buckley Jr. (Random House: $19.95; 276 pp.). Who would suspect that behind the erudite and cosmopolitan exterior of political commentator William F. Buckley Jr. there beats the heart of an unabashed, swashbuckling, romantic? Anyone who has read any of his eight previous novels featuring CIA superagent Blackford Oakes simply takes it as gospel. In Oakes' latest adventure, he shares the spotlight with Tucker Montana. Tucker is every bit as brilliant, hard-living and fearless as his mentor Oakes, but he has a couple of character flaws (one of them being an unbridled libido) that Oakes realizes could--and that ultimately do--pose serious problems. The story is set in Vietnam in 1964; Lyndon Johnson is facing both a worrisome presidential campaign and a rapidly deteriorating military position. "Tucker's Last Stand" is a fascinating footnote to history made even more engrossing by Buckley's insights on the behind-the-scene political jockeying in both L.B.J.'s and Barry Goldwater's campaigns.