1957 Hardy Boys Book. The Ghost At Skeleton Rock. Hardcover.
Cover Image (under jacket): 835 +/- 35 ppm lead.
Interior pages: 74 +/- 12 ppm lead.
Tested with an XRF instrument.
Personally I avoid letting my children use vintage books, because they almost always test positive for lead (plus we live near Powell’s, one of the largest book stores in the world (!), so that makes it easy to go find a new, lead-free copy!)
Here’s my affiliate link to a new lead-free Hardy Boys book set. If you make a purchase using one of my Amazon links I will receive a small commission for the referral at no extra cost to you.