怎么可能修饰 book 呢,她在树下读书,很合逻辑；她在读树下的书,不合逻辑.书应该在她的手里才对啊.她在树下,书在她手里,她不在树下怎么读树下的书?
硬要那样说也行：She is reading the book which was found under the tree.