The Invisible Intruder (The Nancy Drew Mystery Stories) in paperback format by Keene, Carolyn. Published by HarperCollins Children's Books in 1988. Available in Used - Very Good condition.
Carolyn Keene is the pseudonym of the authors of both the Nancy Drew Mystery Stories (and a spin-off called River Heights) and The Dana Girls Mystery Stories, both produced by the Stratemeyer Syndicate.