I've got a soft spot for dictionaries, especially those that focus on some specific topical lexicon - lists of medical, artistic, archaic, slang, or spicy words, as examples. Language, as far as I'm concerned, is the basis for society as we know it, and the breadth and depth to which we develop and record the words we use reveal a great deal about how and why we communicate, the time and place we live in, and what elements of that existence are important to us. 

Cheap books can be an incredibly dangerous delight. It's one thing to walk out of the Brattle parking lot with as many $1 grabs as you can carry, but in these isolatory times, the internet makes all sorts of 'deals' only a PayPal click away. It's all a collector can do to keep within their budget. That said, my most recent Craigslist purchase, at least in terms of historical insight and uniqueness, was worth every penny: two 18th-century farmer’s almanacks, from 1795 and 1797, with much more than meteorological projections packed between and across their covers. 

Brattle Book Shop, Boston. Anyone who's been know it's simply impossible to visit and leave empty-handed. From my perusal of the $5 carts in the parking lot outside the shop yesterday comes this luscious volume from Samuel Johnson's The Lives of the Most Eminent English Poets with Critical Observations on their Works, printed and published in London in 1781.