Off to London for the weekend. Any decent bookshops near St Paul's?

Not really, I'm afraid. There's a Waterstones at Ludgate Circus, which might be open. Then there are some of the finest legal bookshops around on Fleet St, but I doubt they'd be open at a weekend.

For books in town, can I recommend Hatchards on Piccadilly?

You certainly may.

Don't forget Daunt Books – none at St Pauls but Marylebone I think and several more west.

Charing Cross Rd and (especially) Cecil Court, between Leicester Square and Covent Garden) for anything secondhand, antiquarian etc etc. Far more interesting than the chains (and I include HMV owned Hatchards in this!, sorry cfh)