Hello and welcome, Claudia! My name is Doris.
I have been an antiques collector, dealer and appraiser for over 56 years.
I will be pleased to help you.
I researched your book before I decided to answer.
A 1930, Grossett & Dunlap copy of The Hidden Staircase by Carolyn Keene would have an estimated retail value of $1500-1700 assuming some condition issues and depending on sale location.
If it were in top condition with a good dust jacket, it would be worth much more. I have been told by book dealers that condition has a great deal to do with the value of an old book.
In general, a private seller to a dealer, via consignment or at auction can expect 30-50% of estimated retail value.
Insurance replacement values are usually about 10% more than retail values.
I hope I have helped you.
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