Shooting Stars Everywhere — Martina Wildner

The first anonymous letter arrives on July 23rd, Victor's 13th birthday. Which is good, because it gives him something to write about in the journal his father gave him as a present. Soon Victor is writing down everything that happens as he tries to solve the mystery of the letters, conquer his fear of jumping off the high diving board at the pool, and figure out what is going on between his divorced parents.
Things change when he meets D, however. A good diver and an assertive detective, she helps Victor to stop writing his life and start living it.