Read the hornbook if you have time (which you won't) or to get your bearings when you haven't kept up with the reading and/or have kept up with the reading but still feel lost. Or, it can be helpful if you have the Chemerinsky casebook, your prof. assigns you a ton of reading, and you don't have much time. The hornbook is organized in the same order as his casebook pretty much so you can get a good summary of the less important cases and then figure out what case or cases you should focus on reading (or just substitute the hornbook reading for your casebook reading if you don't have time.) Or never read it. It really depends on what you feel comfortable with (which is hard to figure out your first semester.) Whatever you do, don't get advice from your classmates and don't put to much stock in what other people tell you to do. Just tell yourself that whatever it is you're doing, you're doing it correctly and everyone else is either a time-wasting moron or a lazy moron who doesn't spend enough time studying. Self-delusion will do wonders for you in law school.