The Turret (1963 book)

The Turret is the 3rd novel in The Rescuers book series written by the late British writer Margery Sharp
=== Plot ===
The fabulous Miss Bianca has resigned, perhaps prematurely, as Madam Chairwoman of the Mouse Prisoners' Aid Society. In her honor an 
enchanting water picnic is held at the lily moat beneath a ruined turret in the park outside the city. While the band plays Handel's Water Music, the ever observant Miss Bianca discovers that someone is being held prisoner in the old turret.
It turns out to be the wicked Mandrake. No one, not even her dear friend Bernard (who in the course of events may be forced into a most unfortunate marriage), is the slightest bit inclined toward rescuing that scoundrel--in spite of Miss Bianca's conviction that he has reformed. Adventures fraught with danger follow as Miss Bianca, alone, pays a midnight visit to the turret to ponder Mandrake's penitence and make plans. Her plans come to include one boy scout, a precocious mouse named Shaun, and the magnificent stallion Sir Hector.