BET, HarperCollins launch online venture. HarperCollins and Black Entertainment Television yesterday launched an online magazine specializing in African-American books and authors. By year's end, the magazine, called Amistad Bookstore, will include a retail component.Amistad Bookstore is part of BET's online portal, BET.com, which attracts an estimated 1 million monthly visitors, the companies said yesterday.The site will feature reviews, interviews, excerpts and online chats with authors. An advisory board will include writers from rival publishers, including novelist Charles Johnson (Scribner) and biographer Arnold Rampersad (Alfred A. Knopf). The announcement comes at a time when several publishers have recently started black imprints and access to computers among blacks is rising,although still well behind that of whites. According to a Commerce Department study released this week, 23.5 percent of black households had Internet access as of August, barely half the rate for white households of 46.1 percent. The black rate was 11.2 percent in the 1999 survey.