Here’s the sign-up page, and here’s an archive of past letters.
The Res Obscura newsletter is sent out roughly once a month (or every few months), featuring around six to ten links to interesting historical writing. The only selection criteria is whether or not I find something to be interesting. Like the Res Obscura blog, the areas of focus include the history of globalization, history of science and technology, drugs and the drug trade, and oddities.
Image credit: detail from the frontispiece engraving of John Bulwer’s Anthropometamorphosis: Man Transform’d, or the Artificial Changeling (London, 1650).