I believe I found the main entry page to their free PDF ebooks here:
http://www.bookdepository.co.uk/deal...lers&promo=100
There's a menu list of various subjects on the left side. If you select a certain subject, the new page will then give further sub-headings in a new menu (again on the left side) that you can select.
Does anyone know: are these all public domain works that are being offered?