Books... most historical buffs have huge libraries and this period deserves no less... Many of these books are directly useful to the reenactor (so sue me), not having so much emphasis on rusted bits of metal and their interpretation as others do. Scholarly works are fine, but not always the best for a true feeling of the period, nor for learning.
And yeah, the military part is "Osprey-heavy"... why? Because they serve a good purpose; not only to give one a good grounding, but also to interest those who are new to the period.