Bridge to Terabithia by Katherine Paterson
Poor Jess is pressured everywhere, to find his place at home, at school. Then he meets Leslie and she changes his life. Leslie is able to deal with the pressures of the real world (cruel kids, competition) and is also able to find great joy in the world of the imagination. Ironically, it is on her way into that imaginary world, while she is swinging across a raging river, that Leslie’s rope breaks and she is killed. Instead of being filled with sorrow, however, Jess takes everything Leslie has given him and uses it to help himself and to teach others.