Frank Dixon and a cast of thousands, that is. But Caves was #7 in the original series, which means it was actually written by Leslie McFarlane, generally agreed to be the top Hardy ghost. I was a huge Hardy Boys fan as a kid, like my dad before me, and around 1974 he handed over a collection of probably 30 books that he had gotten originally in the 50's - Hardy Boys, Tom Swift, and others, such as the Mercer Brothers, which I've never seen again. Recently I've decided it would be fun to collect some of these old books again, so I picked up Caves at an antique shop for $9 - which may have been a bit steep, but better than the $15 they were charging for some old Tom Swifts.
I don't remember this one as being one of my favorites, though, and on rereading I can guess why - there really isn't a villain. The boys and their chums go in search of a man who has lost his memory, and the escaped prisoner from the local jail is more a distraction than anything else. There are a couple of cliffhangers - literally - which are pretty exciting though, and the missing man is discovered neatly.
I've also picked up a Battleship Boys book and a Submarine Boys book, which I'll review if I think of it. Between a couple of programming books, a spy novel, and a C.S. Lewis, my limited reading time is pretty split up :)