IntroToRx.com is the online resource for getting started with the Reactive Extensions to .Net. Originally starting life as a blog series, it has now flourished into an online book. You can read it online here via the website, or get a copy of the Kindle edition for reading offline.
If you have any comments or requests, feel free to add them on the official Rx forums at this post. You can also tweet me @LeeRyanCampbell.