The university standard text is Jonathan Wolff, An Introduction to Political Philosophy. It is a good starting point, though not nearly as comprehensive as it could be. Colin Bird's recently published book of the same name is more comprehensive, but does not seem to be as widely used. My favourite introduction, which I tend to recommend to people, is Phil Parvin and Clare Chambers, Political Philosophy: A Complete Introduction. Unfortunately, it is difficult to get a copy of this on anything other than kindle format. You might also benefit from Andrew Heywood, Political Ideologies: An Introduction.

There are many a great philosophers who also wrote about government, e.g., John Stuart Mill, John Locke, Thomas Hobbes, and many others.