The book starts off with a short introduction to the Notes Designer IDE. Then roughly 1/2 to 2/3 of the book is devoted to going over each Java class available in Notes. The last part goes over java applets, servlets, and applications you can write that use Domino; it also goes over VisualAge for Java, JDBC, and Websphere, but only touches on them. If I were organizing it, I would have put the last section in before the Notes Java classes.
This is probably a good book for Java programmers who are new to Domino. It would also be good for Domino developers who are new to Java and want to know how to write LotusScript agents in Java. The Notes Java classes are similiar enough to Domino's LotusScript classes that most Notes developers can probably transition easily if they learn Java.
I would not recommend it for more advanced Domino developers who have learned Java on their own. In their case, the online help and sample code you can find on Notes.Net is enough.