The British Home Office has published a 2nd edition of Roy Walmsley's World Prison Population List. The publication is No. 116 of the Home Office Research, Development and Statistics Directorate, Research Findings.
The full-text, in pdf format, is available at:
http://www.homeoffice.gov.uk/rds/rfpubs1.html
[Note: You will need to scroll down the page to No. 116 in the list on this page.]
The World Prison Population List gives details of the number of prisoners held in some 200 independent countries and dependent territories. It shows the differences in the level of imprisonment across the world and makes possible an estimate of the world prison population total.