Philippa Pearce 
Minnow on the Say 

It’s a great discovery. Right there, at the bottom of the garden, bobbing on the river, is a canoe. The Minnow. David can’t help wishing he could keep her . . . The Minnow leads him to Adam, and an extraordinary summer begins. Armed with a mysterious, ancient clue, the two boys set out along the river by boat. They’re determined to find the legendary lost treasure, hidden by one of Adam’s ancestors hundreds of years before. But they are not the only people looking for treasure, and soon they are caught in a dangerous race against time . . .