Secondhand bookshops, which used to be one of my vices, are dying out fast. The last time I made a journey partly to visit one it was to Oundle and I found the shop had gone. The best guide I know to those that remain is The Bookshop Guide.
Gladstone Books, however, was there and I can recommend it. It may even be one of those magic shops, as I found there a copy of a book that people have been recommending to me: Thinking, Fast and Slow by Daniel Kahneman,
I have not read it yet, but I am hoping that just having it on my shelves will allow me to absorb its arguments by osmosis.
If you are thinking of visiting Gladstone Books, then chick its website first. Like all good such shops, it has slightly eccentric opening hours.