Reddit reviews Constitutional Law: Principles And Policies (Introduction to Law Series)

See if your law school offers intro to law courses for undergrads. Mine did and it helped a great deal at piquing my interest in law.

Beyond that, I'd read some books about SCOTUS (G. Toobin's "The Nine" is a good, easy read).

If you want to dig into actual con law without reading yourself into a coma, try Con Law: Principles and Policies by Erwin Chemorinsky (http://www.amazon.com/Constitutional-Law-Principles-Policies-Introduction/dp/073555787X)

It was, and is, the Con Law bible for law students. It's heavy on legal concepts but its written well enough that its readable by anyone. I have my copy sitting right next to me on my desk right now. I can't tell you how good this book is for constitutional law 101 type information.

If you really want to be sure, just get the 4th edition used. It surely will be enough. https://www.amazon.com/gp/offer-listing/073555787X/ref=sr_1_2_olp?s=books&ie=UTF8&qid=1486744787&sr=1-2&keywords=chemerinsky+constitutional+law

The Five Types of Legal Argument is a good primer on what types of arguments are used in the courts that generate case law. Chemerinsky's Constitutional Law is an excellent resource for constitutional law, which is some of the more interesting stuff. The Nine is an easy read and a good introduction to the personalities and major decisions of the Rehnquist court and early Roberts court. Dressler's Understanding Criminal Law is another good one; it explains the general architecture of criminal law and its development. Those might be available at libraries near you. If there's a law library in your area, you can always grab a legal encyclopedia (like American Jurisprudence 2d. or Corpus Juris Secondum) and a Black's Law Dictionary and flip around until you find something interesting. And as others have mentioned, BarBri is a good resource.