Brother Against Brother (English, Paperback, Franklin W. Dixon)

A federal witness is being hunted by an unknown hit man. In an effort to warn the witness, detective Fenton Hardy sends Joe on a secret mission. With frightening swiftness the killer ambushes Joe before he can deliver his message. Jow manages to survive but loses his memory!

Joe can’t remember his name or his mission. Worst of all, he thinks Frank is the enemy. Meanwhile a deadly killer lies in wait – selling death on the family plan.
Source- Goodreads.com