This is a Scholastic Wishbone Mystery
From the back of the book:
When Joe Talbot gets a summer job a t Oakdale's used-book store, he's happy just to have a job. But quickly learns that something strange is going on when he narrowly escapes from a burglar one night at the store. More break-ins occur, and Wishbone and Joe put themselves on the trail of the burglar. They soon discover that there is more to the case than just break-ins, and the person behind them isn't interested only in stealing books. There's a secret in the bookshop and a hidden clue to the mystery that lies in the books themselves!
From the back of the book:
When Joe Talbot gets a summer job a t Oakdale's used-book store, he's happy just to have a job. But quickly learns that something strange is going on when he narrowly escapes from a burglar one night at the store. More break-ins occur, and Wishbone and Joe put themselves on the trail of the burglar. They soon discover that there is more to the case than just break-ins, and the person behind them isn't interested only in stealing books. There's a secret in the bookshop and a hidden clue to the mystery that lies in the books themselves!
