J. Dunbabin 
Captivity and Imprisonment in Medieval Europe, 1000-1300 [PDF ebook] 

This book explores the growing importance of prisons, both lay and ecclesiastical, in western Europe between 1000 and 1300. It attempts to explain what captors hoped to achieve by restricting the liberty of others, the means of confinement available to them, and why there was an increasingly close link between captivity and suspected criminal activity. It discusses conditions within prisons, the means of release open to some captives, and writing in or about prison.