Olive's Ocean, by Kevin Henkes, 2003.

Twelve year-old Martha Boyle never really knew Olive Barstow, but after Olive's death in a bicycle accident, she learns that Olive thought Martha was the nicest girl she knew. Martha takes this knowledge, along with other concerns, to her grandmother Godbee's house on Cape Cod for a family summer vacation. Martha's experiences and emotions are sensitively painted as she contemplates her future, her mortality, her malleable family and aging grandmother, love, and creating a monument to Olive. A lovely and finely-focused book of short, acute chapters and careful honesty. Recommended, though it may hold more aesthetic appeal for adults than most middle-grade readers.