My newest acquisition, for $10 from the library's used book sale. The volume is in wonderful condition and consists of collected writings of Johann David Michaelis in German and Latin. The collection seems to have been published in portions over the course of a decade or so, beginning in 1759. I don't foresee myself making any immediate research use of this, but it's one of those sorts of items that I try to be on the look-out for, purely as a matter of maintaining a collection. I think this is the oldest volume I own.