You can master MODX from these excellent publications.
by Bob Ray
Building dynamic websites with the MODX content management platform. The official and definitive guide to MODX—both Revolution and Evolution. All the information you need to use MODX to meet your needs as a designer/developer. From MODX Press.
by W. Shawn Wilkerson
An example-driven guide which effectively walks readers through various aspects of administrating, using, developing, and designing websites.
by Antono John Solar
Building dynamic websites with the PHP application framework and CMS. Simple, step-by-step instructions on how to install, configure and customize MODX.
by MODxフォーラムJapanese Co
(著) Learn how to use MODX, in Japanese. Originally published in 2007.