Oh man, how did I forget about these...
*The Magicians Trilogy - Lev Grossman
The Wheel of Time Series - Robert Jordan
Discworld Saga - Terry Pratchett
*Cat's Cradle - Kurt Vonnegut
*Recommended for an older audience
Barnubus, stop being a hipster and tell us what these books are :P
Just to name a few:
The young warrior trilogy - William Nicholson
Epic - Conor kostick
The Pendragon Adventures - D.J. MacHale
Sherlock Holmes (all of them) - Sir Arthur Conandoyle
*One day in the life of Ivan Denisovich - Aleksandr Solzhenitsyn
*Candid - Voltare
Princess Bride: A classic...
Ideas like this have come up many times in the past and I'd definitely love to see something like this implemented.
My two cents:
The cost of war should be by the hour. Battles and PvP tend to be very short-lived in Herocraft so a couple of hours should be more than ample for a war to take...