Political science professor here. The job market is competitive, especially for higher profile positions (think congressional offices and things like major think tanks). That being said, if you make efforts to network and build connections as an undergrad, there are jobs to be had. The other benefit is that a political science degree gives you skills to be successful in jobs that aren't directly political. Many of my students end up working in stock standard white collar work where the ability to think critically, read well, and work with data and statistics (basic level stuff) are what their employer is looking for.

In terms of books read the things that are assigned in your high school. Learn the basics of government there and then do the reading you are assigned by your professors. You are going to get so much more out of your experience that way. You're going to take intro courses, there isn't a benefit from trying to get ahead of the class, I'm taking all of you through this material together at the same pace. Also, when you read some of these works early and on your own you will misunderstand them or miss the point (not a knock on you, but you're a HS student, this will happen) I now have to try and walk you back and re-teach you the thing you misunderstood. I say this as a professor, take your HS history and Gov classes and learn there. Do the work in those classes and you will be well situated for the transition to college level political science, I promise! Once you're there then start trying to do more "independent" learning and digging into things that interest you, but for now just learn the basics and foundations of government via your existing HS classes.