First port of call for learning more about media queries should be Zoe Mickley Gillenwater’s post “Essential Considerations for Crafting Quality Media Queries”: http://zomigi.com/blog/essential-considerations-for-crafting-quality-media-queries/.
The authority on mobile devices is PPK, and if you want to find out more about physical and virtual pixels, I suggest you start with his article “A Pixel Is Not a Pixel Is Not a Pixel”: http://www.quirksmode.org/blog/archives/2010/04/a_pixel_is_not.html. Wikipedia has a list of common device resolutions and pixel density: http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density.
Patrick Lauke wrote an article about user-controlled DPR, “devicePixelRatio in Opera Mobile”: http://my.opera.com/ODIN/blog/2012/07/05/devicepixelratio-in-opera-mobile. Matt Wilcox’s article “The Responsive Design Process” has a good glossary of key terms as well as plenty of practical advice on the design side: http://mattwilcox.net/archive/entry/id/1078/.
Read more about the way that different browsers round decimal places in John Albin Wilkins’s post “Responsive Design’s Dirty Little Secret”: http://www.palantir.net/blog/responsive-design-s-dirty-little-secret/.
Paul Irish’s blog post “box-sizing: border-box FTW” sets out his reasons for applying this property globally: http://paulirish.com/2012/box-sizing-border-box-ftw/.
Luke Wroblewski’s book Mobile First is published by A Book Apart: http://www.abookapart.com/products/mobile-first/.
To learn more about content breakpoints, read a pair of articles from Australian web design studio Jordesign (http://www.jordesign.com/blog/responsive-breakpoints-from-the-content-out/) and developer Thierry Koblentz (http://coding.smashingmagazine.com/2012/03/22/device-agnostic-approach-to-responsive-web-design/).
The history of the current favorite responsive images proposal, and latest news on the state of its adoption, can be found on the website of the Responsive Images Community Group: http://www.w3.org/community/respimg/.
Find Matt Wilcox’s Adaptive Images tool at http://adaptive-images.com/.