Barron and Dienes' First Amendment Law in a Nutshell, 4th

"Congress shall make no law...," but on this subject at least, the Supreme Court gets to do pretty much whatever it wants. First Amendment doctrine is notoriously serpentine and challenging. While this nutshell doesn't change that, it does explain the law in a crisp, well-organized fashion, with a good balance of case summary and explanation/analysis. The authors are concise without dumbing down the law. While there are big treatises out there for practitioners or for in-depth reference use (e.g., Smolla and Nimmer), there aren't very many First Amendment study aids out there. I think this one is the best.