The Mystery of the Purple Pool — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're going on an exciting trip to New York City!

The children have a great time visiting the Empire State Building and ice skating at Rockefeller Center. But they discover something very strange at the hotel swimming pool: Someone has dyed the water bright purple! Who would do that? And why?