Way back at the dawn of the 1980s Alan Moore wrote three stories set at the origin of the timelords. They aren't great (they were written before his writing was really even good), but they show more imagination than you see in many writers, and lay in place some interesting stuff about Gallifrey, Rassilon and Omega, as well as playing neatly with the idea of a war across time.
Each one is only four pages long, and you can find them
here,
here and
here.
There's also some interesting commentary on them
here.