The waters of eternal youth / Donna Leon.

Enhanced descriptions from Syndetics:

In Donna Leon's Commissario Guido Brunetti series, the Venetian inspector has been called on to investigate many things, from shocking to petty crimes. But in The Waters of Eternal Youth , the 25th novel in this celebrated series, Brunetti finds himself drawn into a case that may not be a case at all.

Fifteen years ago, a teenage girl fell into a canal late at night. Unable to swim, she went under and started to drown, only surviving thanks to a nearby man, an alcoholic, who heard her splashes and pulled her out, though not before she suffered irreparable brain damage that left her in a state of permanent childhood, unable to learn or mature. The drunk man claimed he saw her thrown into the canal by another man, but the following day he couldn't remember a thing.

Now, at a fundraising dinner for a Venetian charity, a wealthy and aristocratic patroness--the girl's grandmother--asks Brunetti if he will investigate. Brunetti's not sure what to do. If a crime was committed, it would surely have passed the statute of limitations. But out of a mixture of curiosity, pity, and a willingness to fulfill the wishes of a guilt-wracked older woman, who happens to be his mother-in-law's best friend, he agrees.

Brunetti soon finds himself unable to let the case rest, if indeed there is a case. Awash in the rhythms and concerns of contemporary Venetian life, from historical preservation, to housing, to new waves of African migrants, and the haunting story of a woman trapped in a damaged perpetual childhood, The Waters of Eternal Youth is another wonderful addition to this series.