Nate the Great and the Mushy Valentine by Marjorie Weinman Sharmat illustrated by Marc Simont*

My first grader read this book to himself.
Someone leaves Sludge, his dog, a valentine.  His poor dog (Nate hates mushy stuff!)!  Nate is determined to find out who would dare leave Sludge a mushy Valentine!
*Highly recommend.
-Borrowed from the library.
Guided Reading Level: K (end of 1st grade/beginning of 2nd grade)