"LEAN IN." By Sheryl Sandberg. Knopf. $24.95.During the summer between her junior and senior years of high school, Sheryl Sandberg worked as a page for her congressman, William Lehman. On the last day of the session, Lehman introduced her to Speaker of the House Tip O'Neill."I was nervous," she writes in her new book, "Lean In: Women, Work and the Will To Lead," but Lehman put her at ease with a complimentary introduction praising her hard work. O'Neill, who died in 1994, patted Sandberg on the head.