The World of Clowns, by George Spreight
A wonderful book (now unfortunately out of print, but you should be able to find a copy in your library, used bookstore, or Amazon.com 🙂
It does not deal with make-up, costuming, skits, juggling, etc. It does deal with the history of clowning prior to the commedia del arte to the ‘present day’ of the 1950’s. If you’re looking for current information, you’ll have to look elsewhere, although it has a treasure trove of information from the heyday of the circus in the first half of the 20th century.