In The Gathering Storm, Verin gives Mat a letter with precise instructions to either open it ten days after he reaches Caemlyn, or to wait thirty days for her to meet him there. Mat notices that she has more letters in her bag. [TGS 36: The Death of Tuon] Who are they for? What do they say? And why write letters, then ask people to wait to read them?

It's safe to say that at least one letter was probably for Perrin. After all, Verin knew she'd be meeting a ta'varen that wasn't Rand in the village of Trustair, and that leaves very few possibilities. Unfortunately, we can only guess at who the others are intended for.

Several have speculated that Mat's letter includes the location to the Horn of Valere, which he will need at the Last Battle.  Verin was charged by Siuan Sanche with hiding the Horn, presumably somewhere in the White Tower after the events of Falme.

Another intriguing possibility is that ALL of the letters were for Mat, but were dummies.  Verin might be prevented by her Black Oaths from knowingly handing Mat a letter full of information that might betray the Shadow.  So she wrote one letter with the correct instructions, along with several decoys, mixed them together, and then trusted to the ta'veren nature of whoever she was meeting that she would draw the correct one.

As for why Verin asked Mat to wait, it's quite simple. This is her failsafe. She plans to go to the White Tower and either free herself from her Oaths to the Black Ajah, or hand her notes over to someone trustworthy and commit suicide. As Verin says in The Dragon Reborn, "Hope for the best and plan for the worst. That way, all your surprises will be pleasant."  If she's unable to free herself from the Oaths, Verin still has unfinished business. These letters are likely instructions to do things she will be unable to do herself in the event of her death in Tar Valon.