Pick of the paperbacks; Onward and Upward in the Garden, by Katharine S. White. New York: Farrar, Straus & Giroux, Inc. $6.95.

In this group of 14 pieces originally printed in The New Yorker, the author takes her readers into the commercial-exotic world of the seedman's catalog. While Mrs. White never pretends to have been anything more than an amateur hosticulturalist, she is both modest and opinionated. She takes potshots at what she thinks is misguided commercialism.

Mrs. White feels a taste for gardening and for writing frequently coincide, and she is an average-to-good-exponent of the art. She can get happily carried away -- by an extraordinary fragrance or by the color and form of irises, but 34 years of editing for The New Yorker somehow stiffens the flow of her style.