Web Components are quite new as of this writing, so not many resources are around. The developer-friendly introduction written by the spec authors should be your first stop. Next might be Eric Bidelman’s presentation. Both resources are helpful in learning the core concepts. You can find them at http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html and http://html5-demos.appspot.com/static/webcomponents/index.html (you may need to use Google Chrome to view this correctly).
The Shadow DOM is the best-implemented piece of Web Components and, as such, has more online documentation. Both Sitepoint and HTML5 Rocks have clearly written explanations of the topic, which you’ll find at http://www.sitepoint.com/the-basics-of-the-shadow-dom/ and http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom/.
If your browser doesn’t support custom elements, consider X-Tags, an experimental library created by Mozilla that replicates the behavior of custom elements and has an extensive registry of prebuilt components: http://x-tags.org/.
Internet Explorer 10 was the first browser to implement CSS Regions, so their documentation is useful for covering the basics. See http://msdn.microsoft.com/en-us/library/ie/hh673537%28v=vs.85%29.aspx/.
CSS Exclusions are also in IE10, so their documentation should be the first point of call again. Once you’ve finished there, check out some of the demos from Adobe. See http://msdn.microsoft.com/en-us/library/ie/hh673558%28v=vs.85%29.aspx/ and http://adobe.github.com/web-platform/samples/css-exclusions/.
MDN has the best documentation of feature queries, although the API is currently undocumented. See https://developer.mozilla.org/en-US/docs/CSS/@supports/.
As I write this, the only place to learn about Cascading Variables is in the draft specification at http://dev.w3.org/csswg/css-variables/.
If the Box Alignment proposal is still ongoing as you read this, you can follow its progress at http://dev.w3.org/csswg/css3-align/ and find the Line Grid proposed spec at http://dev.w3.org/csswg/css-line-grid/.
Håkon Wium Lie and Chris Mills wrote a very nice introduction to CSS pagination in their article “Opera Reader: Paging the Web”: http://people.opera.com/howcome/2011/reader/index.html. For more on Pagination Templates, see the Adobe Web Platform blog at http://blogs.adobe.com/webplatform/2012/05/31/pagination-templates-in-css/.