THE BELLY DANCER IN THE BARREL OF OIL (Libra: $10). The temptation burns eternal, alas, to write a hilarious novel about a madcap adventuress along the lines of "Auntie Mame." And--again alas--Rebecca Newman in her slim novel has yielded to this old siren song. It isn't that Newman doesn't have a light touch and peels off a fair share of passable one-liners, but there is a matter of credibility involved here. A widow, with two young ones underfoot, who turns to a belly-dancing career and ends up (don't ask how) in a Middle East harem and involved in an international terrorist plot? It's a nice try, as the saying goes, but Newman's heroine emerges, unfortunately, as the sort of dip that every mother fears she may someday end up hiring as a baby sitter.