It feels a bit odd saying "reward me for being your customer", but in terms of marketing, giving away a 'free' ebook with purchase does seem to be the best strategy. And Manning does seem to be leading the pack in this respect; even though the price difference between the two 'combo deals' might not be that huge, the way Manning presents it as a given that you'd want a digital copy of your technical book, I think gives them an edge for 'understanding their audience'. Not to mention that just by being on their mailing list, they send you a ton of good deals on a daily basis, with at least one super good deal per month -- all this, along with their consistently long list of 'MEAP' titles, have definitely made them my first choice when purchasing technical books.