The last month I have been very busy writing on The Definitive Guide to Terracotta.
It is a collaborative effort from the Terracotta team each one writing on chapters in their area of expertise. It is a very practical and will teach you how to use Terracotta with the X framework/appserver or for a Y use-case, yet thorough and deep in concepts and theory.
It will be available sometime next year and if you're interested in getting updates on the book can subscribe to: bookupdate AT terracottatech DOT com.