"Wheels: A Pictorial History" by E. Tunis might be worth looking for. We have a copy in the library that is used periodically by people for painting/art references, as is "Wheel's West: 1590 - 1900" by R. Dunlop. Both are dirt cheap on ebay/amazon/Abebooks for used copies.

We have a local man with quite the collection of horse-draw farm equipment and farm wagons. Most are later 1800's/early 1900's but if there's a particular piece you want me to check with him about, let me know. If he has one, I can go take measurements/photos.