The sort of specialization you're citing here is something that you can do when you have a large market available. Perl's market may not be large enough to support entire books on these niche concerns.

The only reason anyone needs a book on games in Java is because there's a demand for Java applet games. The number of people writing games or doing 3D graphics in Perl is tiny, so a publisher would probably not make any money on those books.

The e-commerce book idea isn't bad. I don't think there's anything special about e-commerce (as opposed to other kinds of web apps), but I suppose it could be a sort of general web application book that uses examples from commerce situations.

Of course the Java books I own may reveal why I'm not very insterested in sexy titles: Java in a Nutshell, Java Cookbook...