"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.
While the content is complete, save some changes from my editor, the site is still under construction. Feel free however to start reading what is ready now. The targeted version is 1.0.10621.0 (NuGet: Rx-Main v1.0.11226). Note that Rx has a v2.0 Beta, which has some new cool features. Those features are largely an addition to the v1 functionality, so you are still best off learning v1 before getting too carried away with the v2 features.
While the site is getting its finishing touches, you can be assured that we are busily working away on getting content for the soon to be released version 2.0 of Rx.
What's also awesome is that not only is this a great intro to Reactive Extensions, that the content is easily viewable on the web, but it's also freely available in Kindle/Mobi form too!