David Sparks has – together with Eddy Smith – written a new Field Guide about Markdown. You can download it on the iBookstore or download a PDF file. But I think the best way to read it is via an iPad (iBookstore version).
This book demystifies Markdown, making it easy for anybody to learn. This book includes 130 pages and 27 screencasts totaling more than one and a half hours of video. There is also an additional hour of audio interviews. This book will take you from zero knowledge of Markdown to being a Markdown pro and change the way you write for the better.