See No Evil (Hardy Boys: Casefiles, #8)

Crime always leaves a bad taste.

Frank Hardy's girlfriend Callie Shaw is determined to join him and Joe in solving crimes. So when she bumps into a mailman and mistakenly recovers a top-secret codebook, Callie thinks she's found the perfect case. She challenges the brother detectives to help her break the code. Trouble is, the real owner is deadly serious about getting it back.

Joe is knocked cold during a daring burglary, and when he comes to, the black book is missing. But not before Frank has uncovered a link to a secret network of fraud, corruption, and murder. It seems Callie has stumbled on to a major scandal -- Will Frank and Joe end up taking the fall?