I can't remember how I got the first book (The Secret of Terror Castle) but I loved it and I went on to get many more from the series.
It's hard to remember, from this vantage point, what exactly I found so special about the books.
The three boys (Jupiter, Pete, and Bob) lived in the (imaginary) town of "Rocky Beach" in California. This was close to LA and Hollywood. So this would have made it exotic for me.
And Jupiter lived in a salvage yard, and had a secret "headquarters" hidden by piles of junk, with cleverly disguised entrances with strange code names. I know I loved this.
And the stories were scary. Another big plus. And they were clever and well written.
And my 3 sisters had no interest in them. Again this would be a big plus for me.
The characters were created by Robert Arthur, who wrote 10 of the original 43 books, between 1964 and 1969.
The edition I bought was by Armada Books and seem to have been published in the 70's.
Anyway, I kept the books all these years and I have just finished reading the first book to my own son, who is now 7. He seemed to enjoy it and I have now started the second book. It's quite possible that I am getting a bigger kick out of reading them than he is!
Here is a link to an official website, created by Robert Arthur's daughter, Elizabeth.
Here is the cover of the first book as it appears on my copy:
