I’m a big fan of second hand bookshops, nothing better than a nice quiet second hand book shop and discovering a gem:

You could spend a day around Kings Cross, Camden and Kentish Town visiting:

• As mentioned earlier: Skoob (King’s Cross)
• Oxfam books and Amnesty books - these aren’t ‘political’ bookshops (although they do have political non-fiction and have books of every genre), but just second hand book shops where the money goes to charity (Both Oxfam an Amnesty in Kentish Town - the walk between the two has many charity shops that also have books).
• Judd Books (King’s Cross)
• Walden Books (Camden)

Then another day head to Waterloo and go under the bridge to find the Southbank Book Market. Stalls and stalls of second hand books - I’ve found some decent vintage books here and there’s a stalls that specialise in old-skool sci-fi, westerns and vintage Stephen King editions.