> Was the Three Investigators series the one where there was a kid named Jupiter (Jupe for short) and there was a junkyard owned by the parents (or aunt and uncle) of one of them and they had a secret meeting room with a tunnel through junk leading to it?

That's the one. The link below lists the books in the series, and amazon.com searches on those titles should provide purchasing information. The "Crimebusters" is the spin-off series I was talking about that didn't seem to be quite as fun.

As a rule of thumb, Robert Arthur's and William Arden's contributions to the series were solid with exceptions; M. V. Carey's were variable, and she didn't have as solid a grip on the supporting characters. Nonetheless, the "Missing Mermaid" one, that she wrote, was my favorite. I don't know what I'd say if I reread them all now.

The first thirty were originally introduced by Alfred Hitchcock -- and included him in the stories as a character that the boys would report to in the end. After Hitchcock died, Hector Sebastian took over these duties, which was fine, but what was not was that the original thirty books were rewritten to take Hitchcock out of them, and now Hitchcock's characteristic introductions have been replaced with Sebastian's rather boring ones. I'm not positive how they did all this and retain continuity, because the first book in the series had a rather long section where the boys try to get into the movie studios to meet Hitch -- all that would have been written out -- and in #31, the boys had a similar episode in their first meeting of Sebastian. They should have just left the first 30 alone. Thankfully, although I may never read them again, I own the Hitchcock versions of 29 of the first 30. I'm sure that, with a little hunting, the Hitchcock versions can still be found in used bookstores.

At any rate, you can also find the Three Investigators series condensed on Book-A-Minute Bedtime.