I've just had a general email from them saying they've launched their ebook store. I think they did have some free ebooks before, but now apparently they have over 340000 to buy. I've not yet had a look at their selection, but that's what I'm off to do now.
Relevant bit of the site is here:
http://www.bookdepository.co.uk/category/3390/eBooks
Given they joined MR last year, I'm surprised we've not heard anything from them directly about this!
(Disclaimer: I have no link to the company except as a very happy customer of theirs for physical books)