Someone gave me this book as a kid. I tried to hide the fact that I read "girls" books. Then came the tv show, followed by the girl who played Nancy Drew posing for Playboy (which I found in my dad's closet...)
ND#30. 1953 cover art. A masquerade party turns into a mystery for Nancy when a cloaked man is seen climbing into the second story window of the mansion and things end up stolen. Once again on the trail of thieves, Nancy must undercover the gang who are stealing jewels and art at parties given by wealthy people. When Nancy and George switch identities, George soon discovers that while it is exciting to play amateur detective, it can be dangerous to masquerade as Nancy Drew!