For the underlying JSR303 Bean Validation specification, checkout the JCP website.
User's Guide
TBD...
JavaDocs
The complete set of online JavaDocs for the provider and optional modules can be found online or downloaded from a Maven repository.
API Docs for 0.3-incubating
Online 0.3-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository -
bval-jsr303-0.3-incubating-javadoc.jar
bval-core-0.3-incubating-javadoc.jar
API Docs for 0.2-incubating
Online 0.2-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository -
bval-jsr303-0.2-incubating-javadoc.jar
bval-core-0.2-incubating-javadoc.jar
API Docs for 0.1-incubating
Online 0.1-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository -
bval-jsr303-0.1-incubating-javadoc.jar
bval-core-0.1-incubating-javadoc.jar
Other Resources
Since Apache BVal implements the JSR303 Bean Validation specification, most articles and HowTo guides for the RI (Hibernate Validator) can also be used as a reference.