Secret page alters the contents of a page so that they appear to be something entirely different. The text of a spell can be changed to show even another spell. Explosive runes or sepia snake sigil can be cast upon the secret page.
A comprehend languages spell alone cannot reveal a secret page’s contents. You are able to reveal the original contents by speaking a special word. You can then peruse the actual page, and return it to its secret page form at will. You can also remove the spell by double repetition of the special word. A detect magic spell reveals dim magic on the page in question but does not reveal its true contents. True seeing reveals the presence of the hidden material but does not reveal the contents unless cast in combination with comprehend languages. A secret page spell can be dispelled, and the hidden writings can be destroyed by means of an erase spell.
Powdered herring scales and will-o’-wisp essence.