There are many quirky book stores in London, but I really love the selection that Daunt Books have, they often have bespoke editions which you won't find in other book stores. Also, the people that work there are so helpful and knowledgable. I can never stop myself from walking in!
Donlon Books is an atmospheric little bookstore on Broadway Market (in East London.) They sell rare art books, style publications, and homemade photography zines.