Reading Mysteries for the Kids

I read on a messageboard that Alfred Hitchcock's The Three Investigators mysteries is out of print. Maybe they're available as e-books. As a kid, I got a big kick out of reading them. I thought it was cool for a trio of pals to have their own secret meeting place hidden in the middle of a junkyard. Before TTI came the Hardy Boys. I always liked their dad's name, Fenton. Did anybody else ever read The Happy Hollisters? They ring a dim bell in me. About the same time, I also discovered Zane Grey, but he wrote in a different genre, the Western.