/lit/ - Literature

Unironically this

For a very basic, but still a sufficient knowledge on nationalism, read Fichte (Closed Commercial State) for the core, Schmitt (The Age of Neutralizations and Depoliticizations) for more contemporary views and Hobsbawm (Nations and Nationalism) for a more critical approach.

All the works I've mentioned are really short and you can finish all of them in less then a week if you want to get into it as fast as possible.
However, that's barely the tip of the iceberg. Read more from the authors above and later expand to their influences and the ones that were influenced by them to get a broader understanding.

Other authors that come to mind would be Anthony D. Smith, Rogers Brubaker, Mladen Schwartz ( >>16020382 utemeljeno), Umut Ozkirimli and Philip Spencer from what I've read of the newer ones. Read non-economical political philosophy from 18th til 20th century, it's all nationalism.