The Trouble with Max (Max & Me Mysteries #1) by Patricia H. Rushford

Max Hunter and Jessie Miller, two sixth-grade girls living near the Cascade mountains, make an unlikely pair: Jessie has leukemia and is bald. Max dresses in thrift-shop grunge and acts tough in school. But their friendship holds life together when everything else falls apart. Then one day Jessie discovers Max’s best-kept secret. Jessie wants to help her, but doing so means risking their friendship and possibly her life.