The title, Annabelle and the Sandhog, introduces two of the book's main characters, so let's take them in order:
Annabelle is a nursing-home aide who befriends the sandhog, in the novel and in real life. We'll learn more about her in a bit.
"Sandhog" is American slang for a person who works underground at an urban construction site.
The sandhog in this story is John O'Malley, modeled after author Ray Paul's grandfather, who made a career of blasting bedrock to carve the foundations of tall buildings in the early 20th Century.