The RDA Primer: a Guide for the Occasional Cataloger, by Amy Hart.
In only 80 pages, Amy Hart makes RDA make sense. Yes, really. She covers the elements of FRBR and FRAD, how they relate to RDA, then summarizes each section with thorough yet understandable charts.
Even though RDA isn’t quite ready for prime time, it doesn’t hurt to start getting used to the new way of thinking. Once you have an understanding of the principles, the summary charts make good cheat sheets.