'Tis the season for Christmas books to do well, with two in the hardcover fiction top 15 and two more among the mass market leaders. For Mary Higgins Clark and daughter Carol Higgins Clark, The Christmas Thief is their third winning collaboration. Publisher Simon & Schuster reports 525,000 copies after two trips to press. The Clark ladies' mystery surrounds the disappearance of a Christmas tree bound for Rockefeller Center, so it is no surprise that mom and daughter will be on the Today show November 30, the day of the festive tree-lighting ceremony. They will also be signing at the nearby Barnes & Noble outlet. Both are scheduled for tristate signings through mid-December. Mary Higgins Clark has written 23 suspense novels since 1975; total sales in the U.S. alone are more than 70 million copies.