Summer of the Swans, by Betsy Byars

Our protagonist is 14-year-old Sara. It's summer, and she feels generally discontented. Her feet are too big, her hair's not right, and she's made the mistake of wearing orange shoes, which only highlight her foot problem. She is also fiercely protective of her younger brother, Charlie. Charlie was 3 when he had two terrible illnesses, one after the other. After which, he was never the same, and never spoke again. Nevertheless, he and Sara have a special relationship. When Charlie goes missing one night, Sara knows that he probably went to see the swans by the lake, and must have gotten lost in the woods.

The whole book spans only about 36 hours, and yet in it we see Sara mature by years. She learns to acknowledge love, to humble herself enough to apologize to a classmate she's misjudged, and to stop focusing on her looks long enough to let the rest of life in a little. The book is a quick read - I finished it in about an hour - but it packs a punch. There's quite a bit of late 60s slang ("that fink Joe") and some ideas that today's reader might find strange (Sara's solution to the orange shoe problem is to attempt to dye them baby blue), but the real story is definitely the emotional one, which still comes through loud and clear.