For the trivia nerd, little bits and pieces of information on any topic will be vacuumed and absorbed into the system.  I did this as a kid (and still do) by reading all the weird little atlases, almanacs and topic guides that I could find. Publisher DK does a great job of this kind of thing for younger kids with the Eyewitness series: visual guides for every little topic.  They do make adult books, too

The Book of Rule explains the system of government of for every country in the world.  Don't know the difference between bicameral and unicameral?  Democracy vs. Republicanism vs. Theocracy?  Absolute Monarchy vs. Constitutional Monarchy?  Is Canada run by the Queen of England? Is their a country where their king is elected? (There is, but the ruler isn't called a king) Who, exactly, elects the US president? (Hint: it's not the people)

I acknowledge that this book is a touch out of date (nearly 10 years old), but generally speaking, the information holds true.  Most countries don't change that much that frequently.