I really wish they had an ebook of the book Good Omens, which he co-wrote with Terry Pratchett, but no such luck. Regardless, there are A LOT of really good books here! And in an unusual addition, one of the bonus books recently revealed, was his book Ghastly Beyond Belief. Very rare. Humble Bundle claims it was never published, which is not entirely correct, apparently there was a very limited press run and the books fetch a ridiculous price.
The bundle is available for another six days or so.
High Altitude Game Design 