To get how-to recipes for common (meta)programming tasks in Rascal, we refer to the Rascal recipes
To learn how to use the Rascal Shell, go here
To understand the terminology used in Rascal and by Rascal programmers Rascalopedia is a focused encyclopedia of meta programming.
To understand error messages produced by Rascal, there is information on all Run Time Errors and Compile Time Errors.
To know everything about every detail of Rascal syntax and semantics, the Rascal reference manual applies.
The full standard Library is documented here.

Rascal can be extended by loading different (third-party) packages. For example:

Flybytes enables JVM bytecode analysis, transformation and generation in Rascal.
Typepal is a generic name and type analysis framework for programming languages and textual DSLs.

Further Reading