Moon Over Marisol Omnibus Vol 6 (Books 16-18). Moon Over Marisol, #6

Marisol has carried a raven feather, a warm cherry pie, a jar of eggroll ingredients, a penguin pebble, a dead decoy key, a puzzle piece, a three-legged wooden horse, and a cat who has died more times than either of them can count. Now the shelf is almost full. The letter has arrived. The number is nearly readable. Three moons left - a petty bay that swallows castles, an old band held together by grudges and ghost musicians, and a final exam that reduces everything she's done to bubbles on a scan-tron sheet.
This is the last volume. This is how it ends.