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50% off is not the same as buy one, get one free. Let's say each Ebook costs $10. On a "buy one, get one free" deal, I have spent $10. On a 50% off deal, I have spent $5

I recommend joining the mailing list too. There's one book on sale nearly every day, and a category (e.g. Python, Ruby, data mining, etc.) every few weeks. If you aren't in a rush to buy your books, keep a wishlist and pick them up as they go on sale.

Thanks for the recomendation, I just bought the book. I am trying to learm more about visualizations (and D3). Any other recomendation?

The cupboard seems dry in terms of D3 right now. But there seem to be quite a few good books with R. The ggplot2 book is great, but I don't think it's on O'Reilly http://www.amazon.com/ggplot2-Elegant-Graphics-Analysis-eboo...