If you buy a lot of (perhaps too many) #books but also want to give money to #Oxfam, I can really recommend their on-line bookshop - I use it a lot & always check there before looking elsewhere for a second-hand book. The site is well-organised, easy to search & use, and you are helping people across the world (even as you indulge your guilty habit of buying yet more books).
Have a look, I'm sure you will not be disappointed!