While we're praising O'Reilly, I should mention Safari Books Online. I'm sure you've all seen the promotions for the service in any O'Reilly book you purchase. I'm a subscriber, and it's a great service. I get electronic access to almost every book in the O'Reilly library, plus books from a number of other tech publishers, all for a little over $20 a month ($45 if you want to get rid of the 10 books/month limit).
It's been great for when I need to review some new technology (e.g., HTML5) but don't want or need to read an entire book about the topic in question. And it works great on my Kindle. Also -- and I'm sure O'Reilly is aware of this -- when I find something particularly good on Safaribooksoline, I sometimes end up purchasing the "deadtree" version of such books so I can have a physical book to read when I'm away from the computer/Kindle.