Question

what is the inspiration for the Doric order as described in The Ten Books of
Architecture?

Answer 1

Your best source is the text materials you've been assigned to read. There is much material about the three orders of classical Greek architecture and the influence they've had ever since, but what inspired them seems to be obscure. Read lots and see what you can find if your assigned reading is inadequate to the purpose.

https://www.google.com/search?q=doric+order+art+history&ie=&oe=