Don't buy books in Europe if you can help it.
Currency exchange aside books are usually double what you would pay in North America. No, I don't understand it either, so it's best to bring any reading material with you when you come.
For general guides to Italy I like H. V. Morton's Traveler in Italy series -- although dated his narrative is engrossing, illuminating and enlightening. For up-dated information I suggest you invest your time in any of the Cadogan books.
A good road atlas is a must for driving in Italy, and I recommend Atlante Stradale D'Italia published by the Touring Club Italiano.
As for Siena guidebooks I strongly recommend picking up a copy of Siena: The Gothic Dream. It has excellent text, good maps and organized to allow you to use it as a walking tourbook.
The APT (tourist ) office located at Piazza del Campo no. 56 has excellent city and province guides and best of all, they're free!
There are few histories of Siena in English -- and those are quite dated. Langton Douglas, A History of Siena (1902) and Ubaldo Cagliaritano's The History of Siena (1983) are two that I've found at Feltrinelli's Bookstore in Siena.
One of the better narratives about Siena focuses on the Palio and the Contrada -- understandably so. Still, La Terra in Piazza (1994) by Alan Dundes and Alessandro Falassi is an excellent read and will certainly whet your appetite for getting to Siena.
If you're a serious photographer definitely get a copy of Sandra Lakeman's Natural Light and the Italian Piazza (1987). You'll come away from Siena with a whole new appreciation for Senese architecture and its relationship to the camera.
And speaking of photographs, you might also be interested in several outstanding collections of old photos of Siena available at most bookstores in the city: Siena by Luca Luchini, Siena: fermo immagine by the Archivio Storico Communale di Siena, and Ommagio a Siena by Alinari Publishers.
If painting is more more to your liking you'll love Ambrogio Lorenzetti: The Palazzo Pubblico, Siena by Randolph Starn.
Iris Origo's War in Val D'Orcia: An Italian War Diary, 1943-1944 is a riveting tale about a young wife and mother, pregnant and in charge of dozens of families on a large farm just south of Siena, aiding escaped British and American prisoners-of-war, Italian army deserters,while fending off German occupying troops and all trying to keep the family farm from utter ruin. An American no less. . . This is one fantastic story, all of it true.
Also pick up a copy of D. H. Lawrence's Etruscan Places, particularly if your interests take you to the very heart of Tuscan civilization.
I also recommend Eric Newby's Love And War in the Apennines. An uplifting and wonderful story of Newby's experiences as one of those British POWs hiding in the mountains of Italy where he just happens to find the love of his life.