Maybe one should add a "donate" via paypal or similar service to the 
free online books.

Our publisher (dpunkt.verlag) also once mentionned that the online 
version of
our German ruby book is probably reducing the book-sales, which might be 
true looking
at our webserver's log-files. A popular game seems to use a spide to 
create a pdf out
of the html-pages. Fine with me :-).

Ciao,
    -Armin.