Grails Plugins, Grails + Geb and lots of Spock: Async, timeouts, suites
Groovy Indy, Grails Asset Pipeline Plugin and the incredible creating your first Grails 3 application Guide
Grails Command Objects: Parameter validation, data binding, type conversion and dependency injection.
Application profiles, because there is more to Grails than just Monolith apps
Groovy and XML. Read it, Write, XMLParser, XMLSlurper, MarkupBuilder, StreamingMarkupBuilder ...
Reflect on Grails in big projects, a new Grails 3 book.
Groovy and Grails Video Courses to entertain you during the holidays
Grails Domain Classes and Enums and start your Groovy AST journey
Grails guides. Open yout test reports!
Grais and React. Several Spock goodies