YOUNG GUNS Two book signings today have only the age of their authors in common: Both writers were 17 when their first novels were published, both books sold extremely well, and now both writers are 21 years old and promoting their second novels. The books themselves, however, are quite different. Christopher Paolini, the young author of the new fantasy novel “Eldest” (Knopf), the sequel to the popular “Eragon,” presides over a brunch celebrating the new book’s release. Afterward, there is a signing open to all; only books bought in the store will be signed (today, 10 a.m. brunch, 11:30 a.m. signing, Books of Wonder, 18 W. 18th St., between Fifth and Sixth avenues, 212-989-3270, $35 for brunch includes copy of the book, free for signing only, reservations required for brunch). Later, Nick McDonell reads from his new novel, “The Third Brother” (Grove/Atlantic), about a Harvard student working in Hong Kong and his turbulent family back in Manhattan. Mr. McDonell’s first novel, “Twelve,” was about a young Upper East Side drug dealer (tonight, 7 p.m., Barnes & Noble Chelsea, 675 Sixth Ave. at 21st Street, 212-727-1227, free).

GARDEN CITY The founder and director of Friends of Pier 84’s Manhattan Botanical Garden, Barbara Hobens Feldt, reads from and discusses “Garden Your City” (Rowman & Littlefield), her howto guide to urban greening. Tonight, 7:30 p.m., Barnes & Noble Greenwich Village, 396 Sixth Ave. at 8th Street, 212-674-8780, free.

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