I have seen a few Review Copies or Advance Reading Copies of Three Investigators books.  The one I own is a First Printing of #32 Blazing Cliffs keyhole paperback from 1981.  I paid $1.00 for it at a used bookstore in Louisville, KY in 1998.  The book was probably sent to a newspaper in the hopes that the person in charge of reviewing books would give it some publicity.  Except for some minor shelving wear, the book appears unused, has the Mystery Quiz page with the bookmark still attached, and contains the Review Form which was found loose in the pages of the book.  In August of 2000, a copy of #16 Nervous Lion in very similar condition with the attached bookmark and loose Review Form was auctioned on eBay for less than $10.00.  From a collector's viewpoint, these prices are somewhat disappointing as I would not expect many copies to have survived with the bookmark AND the loose Review Form still intact (the review forms specifically mention the title of the book in which they are found).  The information found on the Review Form is quite interesting:
A rare Review Copy Form