When am at home in the UK, this is where I get most of my licture books (and more!) from!
Little Linguist are a small business (and who does not like to support small homegrown businesses?) whose prices are most competitive and whose selection of books is amazing! You can also guarantee great customer service with them (not that I have ever had any problems...).
You will also be able to meet Catherine from Little Linguist at various conferences around the country. I regularly do!