I got hooked on the Three Investigators when we were living in Singapore, and I had free access to a used bookstore that took everything back in trade, so I was constantly cycling books in and out like it was a library. They were great - I envied them the secret hideout in the middle of the junkyard.

I also got hooked on a series that was delightfully cheesy and very faux-James-Bond-for-kiddies. They were the Race Against Time books, where mysterious Uncle Richard shows up to babysit and ends up dragging his nephew along on grand spy adventures all over the world...and they have to foil the bad guys and get back home before the parents do. Uncle and nephew both have super-advanced spy watches that act much like Batman's utility belt. I don't know that I've ever run across anyone else who remembers them.