The missing chums - Franklin W. Dixon (The Hardy Boys #4)

Goodreads 3.91

Eighteen year-old Frank Hardy and his seventeen year-old brother Joe no sooner accept a 'shantytown' beach investigation from Bayport Police Chief Collig when two of their close chums vanish. After a crime occurs which they partially witness and the matter of the same squatter town surfaces again and again, they wonder if all of Bayport's havoc could stem from that one place.