It's a bad book in a lot of ways (and has one of the more WTF endings in military science fiction I've ever read) but David Weber's Out of the Dark features functional military commands after an apocalypse (Earth's bombed from orbit by an interstellar real estate scam). I love that book despite everything.

There's also S. M. Stirling's Emberverse, where modern technology abruptly stops working and the villains of the first trilogy aren't survivalists; they're members of the SCA.