Is the content of the letter really that horrible? I feel the key to a horror story is to take away the character's sense of control (done by a supernatural influence, in this case), then constantly increase the tension and suspense without letting off the pressure (or if you release the pressure, it is only to emphasize the pressure when it is immediately reapplied). You might find THIS question helpful.
A monster shouldn't be revealed too soon, because a monster seen is less horrible since it is known and a controllable variable (even if ghastly). A letter isn't a monster. Depending on what it says, the contents can either build the fear level of the actual horrible event (like in Knowing), or the uncertainty of the contents of a message can be the tension-builder (what Lies Beneath). In this case, your character is likely obsessed with identifying the nature of the message, and obsession is a kind of loss of control, so your first goal is met. The next key with horror is to constantly be ratcheting up the tension, so how do you achieve it?
There are always alternatives in increasing tension. The true meaning of a message from a supernatural source is open to interpretation. So if you want to, finding the letter ratchets tension, and realizing it really is of supernatural origin turns it up a notch. Then identifying the contents of the letter can be a peak, but discovering there is a deeper meaning can be another peak, and discovering the whole thing is a lie can be yet another. Then finding out it isn't a lie can be still another.
The whole thing can all be true, and still the subtle supernatural message can have a completely different significance in the end like The Ring.
Do what relentlessly keeps adding tension, and the story will build and build.