Sift together flour, baking powder, and salt. In a large bowl, cream butter and sugar; add eggs, beating well. Beat in lemon rind and juice. Gradually add flour mixture, then stir in oats. Chill dough thoroughly (at least 2 hours). Roll level tablespoonfuls of dough into balls and place onto greased cookie sheets, allowing room for cookies to spread. Using a flat-bottomed glass or custard cup that has been greased and dipped into sugar, flatten each ball to a ¼-inch thickness (dip the glass into sugar each time). Bake at 375 degrees F until lightly browned around the edges, about 8 to 10 minutes. Cool on cookie sheet for 1 minute, then carefully move to a wire rack.