Unfortunately a lot of stuff in London is quite expensive.
Much as I dislike them, I think Tescos is going to be your best bet, they have outlets dotted all over central and greater London so you're bound to find one fairly near to where you are staying. Places like Sainsburys also have similar branches, but tend to be a little more expensive.
There are good public libraries all over the place, but you have the British Library on Euston Road (between St Pancras and Euston railway stations) which should keep you going, it's pretty much on a par with the Cambridge University library as the best in the country.
I'm not sure of the precise location of any secondhand bookshops, but you won't find much in the centre of London because of the high rents. You're more likely to find them in the surrounding suburbs, such as Whitechapel, Notting Hill, St Johns Wood, Southwark etc, although the suburbs of London aren't always too nice.
You may find you'd enjoy a good old fashioned London market, of which there are many. Have a look at this website. 
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Yes there are two paths you can go by, but in the long run, there's still time to change the road you're on.