A person is sitting on the ground reading a book.
The book was on the ground.
If a person is sitting while reading a book, it might be in their lap, not on the ground.
exists e2 e1 x1 x2 x3 x4.(n_ground(x2) & on(e1,x2) & Theme(e2,x4) & Actor(e2,x3) & v_read(e2) & n_book(x4) & (x2 = x3) & Actor(e1,x1) & v_sit(e1) & n_person(x1))
exists p1 x1 x2.(n_ground(x2) & n_book(x1) & on(x1,x2))