Recently, I mentioned that I read St. Athanasius' "On the Incarnation" each year during Advent. There are a few other books worth reading each year, at specific times. Here is my list:
Advent: St. Athanasius, On The Incarnation
Epiphany: Bo Giertz, The Hammer of God (If you haven't read the new one with the final chapter translated into English, do get yourself a copy. I always felt the book seemed incomplete and abruptly ended. The final chapter is a beautiful enactment of redemption.)
Holy Lent: Johann Gerhard, An Explanation of the History of the Suffering and Death of our Lord Jesus Christ.
Eastertide: The Apostolic Fathers. (I prefer the Lightfoot-Holmes translation, but you can get the Schaff one for free. It also includes Irenaeus and Justin, which are also good for Easter.)
Pentecost (St. John's Tide): Walther, Law and Gospel.
Again, some seasons I read them voraciously, other times I read them lightly, and pick up where I left off the previous year. There is no exam. You will not be tested. But you will find that your sermons are much easier to write if you get some good devotional reading for yourself. For me, these are the top books for various seasons.