Magus, a Udistrict staple, takes the cake for the best foreign language section hands down. Not only do they have a great selection of foreign language reference but they have more used foreign literature than all of the other used bookstores in Seattle combined. I have no idea who they buy from but they are always getting new stock in French, German, Japanese and other Asian and European languages.
The proximity to the UW gives Magus great English literature and philosophy sections with a high turnover; chances are if you don't see something they might have it as soon as the next week if you luck out. They have a pretty large hold shelf in front so do yourself a favor and call ahead if you are looking for something in specific but if you are just browsing Magus could take away an hour or so of your day so watch out!
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