A good place to do this is on the Horn of Magnus mission. A Tome is a large or scholarly book. In this game they provide you with the opportunity to roll some dice that give you new weapons. After passing the hog pits, enter the city and stick to the right path, you'll see some bushes at a dead end. Go into the gap between these bushes. Then go to the right in this room and go in between some other bushes. The first Tome of this mission will be found there. Once the mission ends, you'll unlock the achievement.