Over the past few months the O'Reilly ebook deal of the day at $10 (although they seem to be testing the water for selling at over $10 at the moment unfortunately) has lead to quite a few acquisitions on my part. Additionally you can register your paper books and "upgrade" to an ebook for $5 on those. All this made a Kindle 3 logical as it brought a vast number of $10 ebooks into parity with their $30 to $40 paper cousins. My points of warning would be as follows - this is specific as the O'Reilly books are not DRMed. You get them in mobi, pdf, apk, daisy and epub. That said, the smaller Kindle I have is almost completely useless for PDFs, don't even think about it, not sure about the big Kindle though. I've had success converting webpages to .mobi plus all the public domain stuff in txt works just fine.

http://twitter.com/Oreillymedia has the deals but http://twitter.com/timoreilly retweets them and is vastly more interesting to follow.