Books like Secret of the Spa
Carolyn Keene
Secret of the Spa is 155 pages, about 2 hours 51 minutes at 250 words a minute.
7 books to read next
How long each takes
Nancy Drew, Girl Detective 1.8 h 
Judy Moody, Girl Detective 3.4 h 
A Matter of Choice 4.7 h 
High Stakes 4.1 h 
The Phantom of Nantucket 3.5 h 
Global Warning 2.1 h 
The Hardy Boys and Nancy Drew Meet Dracula 2.9 h Bar height shows length. Estimated at 250 words a minute.