Beth Kephart's first book, a luminous memoir called A Slant of Sun, published three years ago by Norton, was a National Book Awards finalist and received glowing reviews. For her second, Into the Tangle of Friendship, published last year, she switched to Houghton Mifflin. Now Norton's Alane Mason has wooed her back for a third installment of her ongoing story. It's called Within the Shade Among the Trees: A Love Story, and explores the vulnerability of a marriage against the exotic and dangerous backdrop of El Salvador. West Coast agent Amy Rennert negotiated the world rights deal, and Norton plans to publish next spring.