A new book by the indefatigable Garry Wills, Negro President, suggesting that Thomas Jefferson owed his presidency to an electoral vote that reflected slave power, has been signed by Houghton Mifflin. Wills was researching a book on Henry Adams under contract to the publisher when he became interested in this largely untreated aspect of the Jeffersonian presidency, which he feels could have influenced many of his policies; so he decided to write it first. Executive editor Eric Chinski bought the proposal from agent Andrew Wylie and plans to publish as soon as next fall.