The Vanishing Shadow (A Judy Bolton Mystery) by Margaret Sutton. Originally published in 1932. The copy I have is a new paperback but with slightly dog-earned corners and ugly sticker residue on the back cover. The font is old-style and the book design has a retro feel to it. See here it sells for $18. I’d let it go for $5.
Contact me only if you’re willing to pick them up from Mid-Wilshire or downtown LA.