I have faced recently with this language. Maybe we work in the same company? I also have this book.
Honestly, there is no reason to learn it. There is no free tool to compile it, and much more promising is Verilog AMS, it supported by all commercial tool and recently open source Verilog-A compiler appeared (Open VAF). Even Verilog AMS is rarely used, and I have never seen people used VHDL AMS. I have access to xpedition AMS, and I am not impress by it.
Modelica has open source realization and a lot of examples, if you want to try system level multi disciplinary modeling.