The number of articles doesn't help to determine the number of books, this is true. But Wikisource isn't only books. Wikisource have also a large amount of texts that never constitute a book.

Portuguese and Spanish Wikisources have the "Print Version" project [1], based on the Print Version from the English Wikibooks [2] (English Wikibooks releases .pdf files containing a full book but Portuguese and Spanish Wikisource have rejected to duplicate the current avaiable material on another file type). This may help to download a entire book for works that are books. But MediaWiki have some limitations for general printing [3] and a external tool to export pages from a category and convert then in a downloadable file may be more helpfull.

This isn't the main problem for me. I really rate the Protection policy used in the most majority of Wikisources to protect the integrity of a text. Wikisource is powered by the MediaWiki, a *wiki* content managing system. A fully proofread text is a fully proofread text, not a dead page that never more receive a new category schema [4] or a interwiki for translation or a annotation or a link pointing to some Wikipedia/Wiktionary article for some specific word or a free image found in some edition that helps to understand the text or (...). I'm anxious to see new developments in the "stable version".

Anyway, the number of "articles" is a valid number of general development. These numbers are something like the total pages from a book that a publishing house have released to the general public. You know the total number of "paper" used at Wikisources but you don't know yet the total number of books and others publications that these publishing house have released.