The Hidden Staircase by Carolyn Keene.

Part of the Nancy Drew Mystery Stories published by Grosset & Dunlap.

Original copyright is 1930, but the front flap lists to The Haunted Showboat so this was probably published in 1958.

Hardcover in dustjacket, 216 pages, with a black and white frontispiece.

Good condition.  Unmarked, tightly bound, in a good, unclipped jacket ($1.00 back then), that's NOT a library discard.  The jacket has chips in its spine ends, small corner chips and rubs along its edges.  

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Thanks for looking!

Location: vintage teen series books, 2nd floor.