The Pizza Mystery — 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 helping out at a pizzeria.

The pizzeria is run by their old friends, the Piccolos, who've been having lots of troubles keeping their business going. Working at the little restaurant is lots of fun, but the children wonder...is someone deliberately trying to shut down Piccolos' Pizzeria?