Most Political Science materials fall into the 320-329 call number range.
It's usually best to start with a keyword search, but subject headings applied to relevant books can help to refine your results. Open the item record of a book that interests you. Scroll down to the clickable subject headings, and run another search to explore similar books.
You can also browse the headings associated with a particular topic as a way of getting started with your research.
Suggested Subject searches (use these words phrases alone or in combinaton as subjects):
If you interested in a specific culture or region, try combining that idea with one of the suggest subjects above
A Magnificent Catastrophe: The Tumultuous Election of 1800, America's First Presidential Campaign by 
A Student's Guide to Political Philosophy by 
Goddess of the Market by 
Oxford Handbook of Political Psychology by 
Soft Despotism, Democracy's Drift by 
The Character of Nations by 
The Political Philosophy of Thomas Paine by 
The Tocqueville Reader by