Nate the Great and the Halloween Hunt by Marjorie Weinman Sharmat illustrated by Marc Simont

My first grader read this book to himself.
On Halloween night Little Hex (Rosamond's cat) has disappeared.  Rosamond won't eat her candy because she is so upset.  So Nate and Sludge set out on this night full of mysterious people searching for Little Hex.
-Borrowed from the library.
Guided Reading Level: K (end of 1st grade-2nd grade)