Two Lives of Charlemagne, a Penguin edition of translations from Einhard and Notker the Stammerer. This book contains two period accounts of Charlemagne and his deeds. I'm always a sucker for primary sources, even if I have too many to ever get through all of them. Fun stuff to trawl through.
Fourteen Byzantine Rulers, by Michael Psellus, is a period account of the politics of 11th century Byzantium. See previous comment...
Victorian Parlour Games, by Patrick Beaver, is exactly what it sounds like. A bit off of my usual beaten path, but a useful reference book to have, especially since I do get questions about post-SCA periods fairly often.
Now into the more interesting stuff: the Gastronomic Bibliography is a modern limited-edition reprint of an apparently comprehensive 1939 bibliography of cookbooks through history. Spot checks indicate that it's pretty complete, and it lists some (generally non-English) period cookbooks we don't have yet. So as a checklist, if nothing else, this looks to be a useful addition to the cookbook library.
My favorite of the new acquisitions: A Transcript of the Stationers' Registers, 1554 - 1640, volumes 1 and 5. This is just what it sounds like: a dry, plain transcript of account books. But it's just the kind of book that is immense fun to read between the lines in. The accounts paid for feasts tell us what ingredients and elements went into a typical mid-sized feast, and the prices of those elements. (I note, for example, that hypocras cost almost ten times as much as plain wine, which tells us something about the relative cost of the spices.) All kinds of little details of life can be gleaned from things like the frequent mention of rentals of the "hearse cloth" for funerals. We can even tell that they rented out the guild hall for weddings quite frequently (weekly sometimes, during the wedding season in the summer). And of course, there are the lists of books that they registered, most of which I haven't heard of and many of which I want to look for, in case they are extant. (A 1557 guide to palmistry? Now there's something a lot of SCAdian mystics would love to see.) A delightful source for a dilletante like me.
Finally, the oddest but coolest of the bunch: for ten bucks, they were selling The Rights and Liberties of the Church, Asserted and Vindicated, Against the Pretended Right and Usurpation of Patronage. Okay, it's a religious rant. And it's in truly crappy condition. But it's from 1689! Heck, even the signatures of the various owners down the decades are mostly older than anything else I own. For so little money, there was no way I could *not* buy this one...