Literature.

It's hard to pick just one book. But generally, these are the books I reread often and always keep close by:
1) Almost a complete collection of historical and ethnological works by L.N. Gumilev.
2) "Catastrophe" by A.A. Zinoviev.
3) A collection of articles by Nikolai Ustryalov about National Bolshevism.
4) Books on the history of Russia by G.V. Vernadsky.
5) Books by Mikhail Uspensky (especially about Zhikhar).