There are entire online databases full of reference material. Take a look at these:
Encyclopedia of World Constitutions
K 3157 .E5 E53 2007 (Reference)
Arranged alphabetically by country, this 3-volume set provides detailed information about each nation: constitutional history.
Political Handbook of the World : Governments and Intergovernmental Organizations
JF 37 .P6 (Reference Desk Collection)
This book examines, in alphabetical order, the governmental organization and politics of each country in the world
Statesman's Yearbook : The Poliics, Cultures and Economies of the World
JA 51 .S7 (Reference Desk Collection)
Offers up-to-date snapshots of the countries of the world, including key historical events, population, climate, social statistics, constitution, government, recent elections, current administration, international relations, economy, industry, and trade.