I think the biggest question I had as a Pre-L / One-L was about supplements. In fact, the Lawbitches did a nice post about it on this site -- check the archives.
You're a month into law school and maybe just starting to get the hang of it. So of course, the prof's are ratcheting up the reading assignments and you may start to feel a little overwhelmed. You're looking for an out. You think, f--k it, I'll buy a bunch of supplements.
My thoughts, in no particular order:
"Supplemental material" like commercial outlines and case briefs, aren't as useful toward the end of the semester as you would think. Get what you're going to use early, refer to it often during the semester... even when it feels like you don't have time. Its worth losing an hour of sleep now to know what's going on at finals time. Ideally, you should only have to review at finals... not learn it.
Casebooks can leave you feeling bogged down in minutiae. Supplements are great for big picture before you do your casebook reading... if you do casebook reading. (See discussion of Con Law in prior post).
Don't be the girl who brings commercial case briefs to class, and reads from it aloud in front of the professor when called on. Unless you're in a REALLY big class and can get away with it. There are no true short cuts, but there are ways to be more efficient.
There is no "right" supplemental material -- and what you need varies by class. There is no secret -- Emanuels versus Gilberts? High Court versus Legalines? Doesn't matter, really. I only used case briefs in two classes, for example, and a commercial outline in one.
However, the Examples and Explanations series is terrific in Torts and Contracts, and to a lesser extent effective for Property and Civ Pro. Get these.
Buy supplements used on Amazon if possible.
You don't have to buy the hornbook if its recommended but not required. Its in the lawbrary. You can read it there, when need be.