Vampire Trail.

Danger on Vampire trail is the 42nd Hardy boys book released here, and it seems to be something of an anomaly. There are two stories which the boys are investigating, the first is the wholesale fraud on the Magnacard credit card (a sort of prototype American Express) and the second is the illegal mining of sapphires up in the rockies. The anomaly here is as it's written in the early 70's is this the first story or detective novel solely concerned with credit card fraud? The main story is that Biff Hooper, friend of the Hardys has a bloodhound called Sherlock which they bring on a trip out west celebrating Chet Morton picking up a cheap trailer which turns out was bought fraudulently on a Magnacard. Several incidents take place as they head towards Denver, Colorado and there is a show down of sorts up on Vampire Trail complete with imported Vampire bats. There is a few things here for pedants like myself. Chet has no hobbys in this one. Biff Hooper gets a dog for this one novel. Though I'm pretty sure he had another called Tivoli in another book. And we learn that Mrs Hardy wants a sapphire as its her birth stone making her birthday fall in September. Anyway there isn't too many differences between the two editions. We get no next book at the end and a few terms are changed, trailer is now caravan. You can read both the UK and US versions here and here.