Are there any good books or sites to learn about the structure of how to maintain a nation? Such as what to do to rebuild a country or steps of preparation for war. Reason I'm asking is because the fic I'm writing is mainly about rebuilding a fallen empire from the ground up so I wanted to do some research on how to stabilize and maintain a a government.

3699544
Rebuilding and stabalzing a defeated nation depends on who is doing the rebuilding and whether or not the people will rally to support that rebuilding/stabalizing effort

3699544 The history of Weimar + Nazi Germany is a pretty good example on this. Any introductory school text on the subject should be able to give you some tips.

'Coup d'Etat' by Edward Luttwak is basically a spoof how-to guidebook on overthrowing a government. It's a good way of thinking about a) the different between theoretical power and actual power, b) on how governments sometimes sacrifice on other priorities to maintain stability, and c) the structure of power in a basic government.

I also have some books about how the British administered Hong Kong (my hometown, might have heard about it in the news lately), but that's stuff you're unlikely to find where you live. Basically speaking the British maintained Hong Kong through the government co-operating with local millionaires.

3699544
The Rise and Fall of Nazi Germany and Imperial Japan and the subsiquent post-war occupations of Germany (at least West Germany) and Japan are the best examples, and usually the ones people refer too, and there are plenty of books about the subject. Also look for books related to the rise and eventual fall of Napoleon and the First French Empire, as well as books on the history of South Vietnam (for an example of how the effort to rebuild and stabalize a nation failed)

Not sure about anything to do with restructuring things, though there should be at least some history books about things like the depression and the aftermath of the world wars, maybe things about the civil wars (Brit and US) or the Independence War. Another place you might find some bits and pieces (but I'd take them with a large dose of salt) is some of the RPG source books, things like Gurps Space or ones aimed at building detail about nations or towns since they might have some odds and ends that could be useful. Given most of those are sci-fi/fantasy or meant for games I'd not take them as your primary source though.