The biggest advantage these books have is that they do not have competitors. What I mean by this is that if you feel the need to supplement the documentation that comes with these Sybase (Powersoft) products then either these books meet your needs or there aren't any product specific books for you.
The documentation that comes with these products is pretty good by the standards of the industry (in other words it could be a lot better if written by professional technical writers with a real understanding of what users want from documentation and how to respond to those wants.) I think that most users will find that they need good generic books on Java rather than product specific ones. However Power++ is a little different as you may want a bit more on the use of this component assembler that sits on top of Sybase (Watcom) C++. Unfortunately as you will see elsewhere (Q&A column) Watcom C++ is in need of a new release but Sybase were unwilling to tell me about their plans for this product.