I keep track of all the letters I write in this letter journal. The right side of the page lists letters written to whom and on what stationery, and left side lists one of the pens and inks used (usually the last one used in a letter, as I log the letter just before I seal the envelope). I started listing the pens and inks because over time some of them fade, and I've gone back to old pages and said, "now what ink color was that?"
I don't keep a record of letters I receive, but since I write back to every letter, I can sort of deduce that. I've toyed around with listing letters received on what date as well... might start that up with the next journal.
The outside of the journal is shown here in the bottom left, with my letter opener resting on top of it:
