If a man doesn't call you, he's just not that into you. If he doesn't take you on dates, he's just not that into you. If he doesn't want to marry you, he's just not that into you. The advice is refreshingly and infuriatingly simple, but the makings of an Oprah-endorsed blockbuster book? That's what no one expected. Aside from the counsel, He's Just Not That Into You's success lies in the snappy back-and-forth between its authors: smug married Greg Behrendt, 41, and singleton Liz Tuccillo, 42. ''You need Greg to be that sure of his message, but then you need somebody whispering in your ear, 'I understand how hard being single is,''' says ex-Sex and the City scribe Tuccillo. Behrendt hopes his advice lasts long after the book slips from the best-seller lists. ''I'm psyched because I have two daughters. So when boyfriends show up I'll say, 'You read the book, right? So we won't have any problems, right?'''