You never know what you’re going to find among the digital reference sites of available scanned books. Usually the only free books available are ones that have been long out of print but sometimes you run across one that is current and for some reason being offered as a free download. My latest find is The Classical Orders Of Architecture by Robert Chitham which I think is the best manual available on designing and using the orders.
I found this book over 25 years ago and it went out of print. Chitham wrote an updated edition in 2005 which includes a 96-part module as well as the traditional 100-part system that makes computing the proportions much easier if you’re working in feet and inches.
This book basically replaced Robert Gibb’s Rules For Drawing which had been the standard classical manual for over 200 years.
The proportions are easy to understand and there is an excellent section on the use of the orders as well as a very good glossary of classical architecture terms.
If you only have one book on classical architecture for design purposes, this should be it.
You can find the pdf for download here.