I have a book with writing that has key information. I'm trying to hide it from my players to make it a bit more challenging. I was going to use the Erase spell but can the spell "Erase" even be dispelled or undone. I want them to still be able to discover what the book says. I was also just thinking of maybe polymorphing the book into something else? I can't seem to find a spell that just blurs the writing or at least makes it unreadable.
As Kyle says, this is well within the realm of reasonable challenges for a player party, so you could easily just fiat it. If it makes you feel better, though, you could totally use secret page to do this. You could use secret page to do a lot more than this. Alternatively, the Linguistics skill has rules for forgery and deciphering difficult or obfuscated text.
Tell the characters that the book has been deliberately written in a difficult to decipher shorthand, and uses many archaic symbols.
Mechanics: Use a set of skill based challenges, performed over time.
i.e. character must pass 3 Knowledge Arcana checks DC 20, and is able to make one per day.
This represents the difficulty of decrypting the arcane symbols and the deliberate encryption of the content.
This allows for an experienced Wizard to get it done fast, a moderate skill wizard to succeed eventually, and a novice to never get to the 20 needed.
The Second Darkness AP had a spell Codespeak that might do what you want mechanically.
(see http://www.d20pfsrd.com/magic/all-spells/c/codespeak) Basically the spell creates a code that only the targets of the spell can understand - and while under the influence of the spell you can write in that code language - however when the spell ends the writings are gibberish - but understandable by anyone who is targeted with another casting of Codespeak by the same caster. The spell does, however, note that while Comprehend Languages doesn't work on this code (but does reveal that they are speaking/writing a magical language) a Tongues spell does. As a GM I might either allow that if my PC's were relatively low level (so they get something at low level they can't decipher but when they can cast Tongues they have a way to understand it) or I might further complicate things for the PCs depending on their levels and the needs of my game.
Non-magical codes are certainly another option - and a piece of writing could be easily read without being easily comprehended. i.e. it could be in a somewhat obscure language but be easily read by a speaker of that language or someone with a good Linguistics check (or comprehend languages cast upon them) but the meaning of the passages may be obscured to all but the intended readers. (you can then make this a Player driven puzzle to guess what various fake names or words refer to in the context of your game - or you could make this a character driven puzzle/challenge via further game developments (like finding a codebook on a future adventure)
Make it a magic book tied to specific circumstances ala. Thorin Oakenshield's map of the Lonely Mountain. They can Identify the book as magical, find out the requirements for revealing it's contents, and possibly figure out some ways around those requirements with Knowledge Arcane/Spellcraft/Use Magic Device checks. Then they are on the Path to Adventure! Now they need mermaid tears, the hair of a gorgon, view the page at the top of Mount Pointy on the solstice, answer a riddle, be knighted, learn the secret words of the brotherhood of the bent key, whatever.
Whatever you do tie revealing information with advancing the plot or future encounters.