The best thing about any charity shop is that you never know what you are going to find. I have a well tested route around London that includes all of my favourite charity shops. This Oxfam has a great selection of books and often turns up some interesting clothes too.
The St Bride Foundation is the home of the St Bride Library, an incredible resource of printing history, in the form of books, printed ephemera, and tools from the trade. It also hosts many fantastic talks each year – check their website for details.
The many book stores dotted along this strip can provide hours of inspiration. Aside from those selling new books, several specialise in second-hand and antiquarian books.