I happen to be in the right mood to solve mysteries, again. I have started reading The Secret of the Old Clock by Carolyn Keene, which is the very first volume in the Nancy Drew Mystery Stories, and was published in April 1930.
Yes, it is a pretty old book to begin with, and it is claimed to have sold about 2.7 million copies since its first release.
Unfortunately, I could not find the original text to purchase, and I did not feel like reading a translated version, thus I got an online copy. It is a wonderful feeling to have a computer with an Internet connection, when you need it.