The Mysterious Edge of the Heroic World by E. L. Konigsburg

Amedeo Kaplin longs to discover something; he also wants to find a true friend. William Wilcox is an unlikely candidate, but the two boys find themselves working together on a house sale for Amedeo's eccentric neighbour, Mrs Zender.

Every item in Mrs Zender's mansion has its own story. The boys become caught up in one particular story about a picture, a young boy, an old man and a painful secret dating back to the outrages of Nazi Germany - a story that takes them to the edge of what they know about heroism and the mystery of the human heart.