Profession: Barrister, or how to keep the PCs from gaining something horribly valuable

[The Human Diversion]

Don't read if you're in my Wednesday Night Faerun game!

So we have this boat ...:

The PCs in my home game have managed to capture a small pirate vessel that I had already planned for them to not have; essentially it was originally a diplomatic courier under Neverwinter's flag and the treaty that Neverwinter has with Waterdeep says that if the vessel is ever harbored in Waterdeep and not under the control over Neverwinter it must be returned. The real treasure on the boat is a cache of documents written in a secret code in the captain's quarters that I want to tempt them with but not actually have. These documents are essentially "dirt" on a large number of nobles and city officials in Waterdeep, as the pirate captain was something of a Talon Karrde type who dealt more with information than valuables.

At some point the PCs will make a high enough linguistics check to figure out what they have and even decode the documents. How would a good information broker protect their information in a fantasy setting? Is there a "higher" level spell along the same lines as Illusory Script? Can you attach a contingency to an object so that if someone other than the writer tries reading it, it's ruined?

Essentially, the only bit of information I want them to get is the location of a long, lost dungeon because the pirate captain was just going to trade this away - she's a pirate/information broker, not a dungeon delver. She didn't have that document protected because she was planning on trading it, not using it to gain leverage and/or more information.

[Brambleman]

This is too easy. A simple fire trap spell will burn the documents up as soon as they trigger it. This will leave them with whatever "conveniently" survives. The trigger is breaking a seal or opening a folded letter.

[Imbicatus]

The second paragraph on each page reads "I prepared explosive runes this morning"

[Bigdaddyjug]

Yeah, this does sound like a perfect opportunity for the classic explosive runes trap. The only problem with this is that one of the casters in your group is almost certain to cast detect magic on the documents before they start to read them. Sounds like you'll need a permanent heightened misdirection cast on the documents as well.

[Ascalaphus]

You could use a variant of Explosive Runes, that only explode if you don't speak the password. If you do say the password, the runes are the valuable text.

[The Human Diversion]

Is there a game-mechanic way so that if detect magic is cast and then focused on the documents, they burn up?