Dover in June released a paperback of the first Hardy Boys book, The Tower Treasure, originally published in 1927. That suggests to me that it is now in the public domain. Are we there (1927) yet?
Here is the Kindle link.
https://www.amazon.com/Tower-Treasur...dp/B0B3C991NF/
Here is the Dover link.
https://www.amazon.com/Tower-Treasur...dp/0486849856/
You will note that some of the reviewers are saying that they ordered the 1927 edition, but received the 1959 edition instead. So I don't know what you will receive if you order the eBook, now going for $5.20, 42% off.