the world's largest variety of books and plans for building traditional horse drawn vehicles, wagons, carts, carriages... is at one website

Just a few examples

I learned of this when corresponding with Susan Green who did a lot of research on "Horse drawn Military, Civilian and Veterinary Ambulances."

https://horseandcarriagebooks.wordpress.com/
http://www.wildhorsebooks.com/HorseDrawnVehicles.htm