I've read #s 1-30 of the original Nancy Drews (before they were revised-- and poorly so--in the '60s.) This last one by Mildred Wirt Benson is by far her best. The plot is well crafted, unlike some of her previous titles that rely on coincidental meetings to carry the action along.
Nancy is a thoroughly modern dectective. She methodically investigates, searches for clues, tracks down their origins, consults with the police department and interviews suspects and witnesses. At one point I was reminded of the tv series, CSI, when she has a chemical analysis made of the ink writing on the lining of a hooded velvet mask. How modern is that? Another clue is a metal charge plate Nancy goes back to find along the train tracks. Just enough of it that hasn't been smashed by the train identifies its issuer as Taylor's Department Store. Wow! You could charge goods way back in 1953? Who knew?
The characters are more three-dimensional. George, in particular has more to say than her usual, "Hypers!" She shows great courage and then weakness after a harrowing encounter with the vicious Velvet Gang, as a modern girl would. Members of the gang are also a little more rounded out than Benson's typical bad guys. Besides the usual knock-em-out bang to the head, these thugs also rely on ether to render victims unconscious. And at least one of the gang is a reluctant evil doer. All in all, this Nancy Drew may be Benson's last, but it's obvious that she saved the best for last. If you haven't read Mask yet, be sure to pick up the earlier, unrevised edition.