How to handle style in the APT markup language?

APT doesn't currently support style. It is in the roadmap.

How to export in PDF?

An iText module exists using the iText XML document. Unfortunately, the iText team discontinued the XML to PDF functionalities.

A FOP module is currently in development in the Doxia sandbox. You can get the source here.

Is it possible to create a book?

Doxia also has a fairly simple tool for writing books. It comes complete with a Maven plugin to produce PDFs, LaTeX documents and Xdoc for direct integration in your Maven site.

The Doxia Book code is still in the Doxia sandbox, but it is fully functional allthough limited.

See Writing Books in Doxia for more information.