This page has general references about telephony (E.164) numbering principles and practices. The web sites of most regulators and many service providers are also useful. Some more specific references are on the reports page.
Principles
World numbering developments, a revised and extended version of Numbering trends - a global overview
An introduction to numbering: a description of the NANPA approach to numbering for North America also relevant elsewhere
Learning About Numbering: an on-line course by the Australian Communications Authority for the ITU Asia-Pacific Centre of Excellence
ECC/ERC/ECTRA Reports (ERO): links to reports for CEPT, including several about numbering for Europe also relevant elsewhere
Documents & Reports: Numbering (ITU): links to reports
ITU-T Study Group 2 (Study Period 2005 - 2008): links to documents for the ITU study group responsible for numbering
Practices
National Numbering Plans: information about national numbering plans deposited by countries with the ITU
World Telephone Numbering Guide: area code details and recorded changes in national numbering plans
numberingplans.com: area code details and on-line tools for national numbering plans
numberplan.org: area code details for national numbering plans