This is a lot of 29 vintage hardcover books in 24 volumes (5 of the Nancy Drew books are Twin Thrillers) as follows:
Hardy Boys by Franklin W. Dickson from the 1960's and 1970's
The Tower Treasure
The Secret of the Caves
The Mystery of the Flying Express
The Secret Warning
The Hidden Harbor Mystery
The Great Airport Mystery
The Secret of the Old Mill
Nancy Drew by Carolyn Keene from the 1970's:
The Mystery of the Ivory Charm & The Whispering Statue
The Hidden Window Mystery
The Secret of the Old Clock & The Hidden Staircase
The Bungalow Mystery & The Mystery at Lilac Inn
The Clue of the Broken Locket & The Message in the Hollow Oak
The Sign of the Twisted Candles & Password to Larkspur Lane
The Secret of Mirror Bay
Cherry Ames by Helen Wells:
Cherry Ames Department Store Nurse (1956)
Cherry Ames Camp Nurse (1957)
The Bobbsey Twins by Laura Lee Hope:
The Bobbsey Twins (1904)
The Bobbsey Twins At Cherry Corners (1927)
The Bobbsey Twins and Their Schoolmates (1928)
The Bobbsey Twins Solve a Mystery (1934)
The Bobbsey Twins In The Mystery Cave (1960)
The Happy Hollisters by Jerry West:
The Happy Hollisters on a River Trip (1953)
The Happy Hollisters and the Secret Fort (1955)
The Happy Hollisters and the Sea Turtle Mystery (1964)