Local Independent Bookshop. I hope your first choice is always to buy from a local independent bookshop. They can offer good deals and are usually able to get any book you want.
Online Bookshops. Prices vary all the time so these are linked to Andrew Norriss’s books for you to check through quickly.
David Fickling Books (for Jessica’s Ghost only)
Amazon (not necessarily the cheapest!)
Buy a signed copy from Andrew Norriss
Send me a message in the contact form below, which is not public, and I’ll sort out the details with you.