I like the letters idea! Speaking in character through NPCs can bring a lot of flavor to a setting.
As for hidden beneficial rules, that's sneaky of you...I approve.
I do know that all of the players did read the letters sections of the Primer, because topics were brought up in the letter were mentioned in the campaign and they knew right off what I was discussing. So the letters approach was a hit for me. I will definitely take this approach for the next campaign and the PPT.
The way I made each letter
unique though was to change the font, but each font had to be readable.
Another thing I did though not necessarily at the beginning of the campaign was to write a short story about certain events going on, because one of my players asked for a recap for a couple of sessions. The story was only two pages long, but the players really digged that too. The short story was a way to give the players a recap of some previous adventures to help them to keep track. I also wrote another portion of it from a journal perspective of one of the NPC's that they were dealing with.
I know in our campaigns, not all our players are taking good notes of the great details we as DMs give them. This kind of story recap helps them keep it as a handout and as DMs we have full control as to what really important stuff about our campaigns we want them to pay attention to for later or for clues. I like to keep it short though and limit it about three pages at the most. Anything longer than that, unless you're a brilliant writer, you're likely to put your players to sleep (I'm guilty of that). Also, I only want to clue in a few details at a time.
Of course, the last thing is that when any DM gives their players a handout, story, primer, or so on,
the DM must edit it and use proper grammar. I know when I get stuff at a con game and it's filled with typos, bad grammar, and I see that there is pages of this stuff, I just give up and try to wing it. My home game players are no different. If I've rushed-job the handout, they stop reading after the second or so paragraph.