(Culled from Part 1 – see Part 2 for rationale).
Sticking with the historical ones (and not a true post apocalypse in sight yet), I would now like to recommend David Drake’s and Eric Flint’s Belisarius series. In the far future, 2 sides (good and evil) wage a war by each sending back an artificial intelligence to the Roman era. One (good) to the Roman general Belisarius, and one (evil) to the Malwa Empire of of India. This provides each ancient socieity with new information and training to try to change the course of history towards more enlightenment, or towards more darkness. The story follows the intrique and military battles and the tactics and technology introduced by the AIs.
Wikipedia has a far better description than I can manage.