Powell's is the four-hundred-pound gorilla of bookstores. If it were an apartment building, hundreds of people could live in the space comfortably. It's got three-plus floors of books on assorted topics and takes up an entire city block. Pretty darn huge. As for selection, I've found Powells good for acquiring obscurities as well as highly popular texts. It's not very good for general browsing; pick a section and stick to it. Used-book prices are also a bit high, and make sure you pick up a map at one of the info desks; I can imagine someone getting lost in there for weeks, surviving off the remains of people's coffee cups!
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