Quinto sells second-hand books. I spend a lot of time here and have found some inspiring books over the years, both at Quinto and the other shops on this road. Koenig Books on the corner is good for art books, too.
Free natural ponds, open all year round (check online for varying opening hours according to seasons). Get an absolute get away feeling right in the heart of London, a treat if you enjoy swimming with ducks. Amazing park to enjoy a few games of cards and a walk around afterward too...
I am definitely not precious about the materials I use to make my work, but every so often I find myself in need of something more specialised. L.Cornelissen & Son is a beautiful and well stocked art supply shop with extremely knowledgable staff.