| You can buy your books whenever you like. They're returnable for at least the two weeks of the quarter (and perhaps a little longer), so don't worry about whether or not you're going to drop the course. There are a couple reason why you might want to get them sooner rather than later, however. The used books, which are cheaper, tend to go pretty quickly, and they're cheaper, so if you're looking for bargains its to your advantage to get there as soon as possible. The second reason is that, depending on the course, sometimes the books will sell out, and then you'll have to wait about a week for them to get more in.
They'll keep the books on the shelves for at least the first half of the quarter, but after the first week or so, if they run of something, they'll probably order it on a case by case basis, so if you check back a few times and they're still out, ask the people at the desk downstairs and they'll help you order one (ordered books are not returnable, however, so be sure you're not going to drop the class). Most likely, you'll have to buy all the books listed for your IHUM. Occasionally, your TF will tell you that you don't need certain books, but you can always return them, so no harm done if you buy them before the first day of class.
One hint is to go buy your books in the morning. The line to check out can get pretty long in the late afternoon/early evening, especially during the weekend when most upperclassmen are back and wanting to buy their books. | 
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