Every third Saturday of the month, the Crook County Library in Prineville, Oregon has a used book sale in the lobby. (Well, every third Saturday except for in July when they have the HUGE sale over Independence Day weekend.)
Luckily for me, I was heading over to Bend for the weekend, so had a chance to swing through Prineville and stop at this month's sale. Not bad. The selection wasn't enormous, but there were several long table stacked three rows deep, mostly donated (not ex-library copies), mostly hardbacks, and mostly in good shape. Hardbacks were all $1 and paperbacks were 50 cents. The newer books were mostly mysteries; the older stuff was more varied.
Among other gems, I picked up a copy of The Silent Spring in very good condition. It's not a book I'm all that excited about, but it is on the Modern Library's list of Top 100 Nonfiction Books of the 20th Century, so I will get around to it someday.