Find original Nancy Drew titles. Convert the missing letters represented by the Xs into a location. Use the word you get as your certitude answer (no spaces) and you'll be rewarded with the cache location.
1) The Secret of the Old _ _ _ _ X
2) The Secret in the Old X_ _ _ _
3) The Clue of the _ _ _X_ _ _ Chimney
4) The Mystery of the Moss-Covered _ _ _X_ _ _
5) The Clue in the Old X_ _ _ _
6) The Hidden X_ _ _ _ _ _ _ _
7) The Sign of the Twisted X_ _ _ _ _ _
8) The _ _X_ _ _ _ _ _ _ Statue
9) The Secret of Red _ _X_ Farm
10) The _X_ _ _ _ _ at Lilac Inn
11) The Clue in the _ _ _X_ _ _ _ _ Wall
12) The Message in the Hollow X_ _
I'm told the Xs spell out her location, run it through certitude and if you get it right you'll be rewarded with cache coords.
Good luck!