One approach would be to release the text of
books a suitable time after their publishing
date- perhaps two years would be appropriate.
The computer field is fast moving, and the
value and sales of older books is less as time
passes.

Another idea would be to release the text of
original editions of books a short time after
new editions are published. For example
Programming Perl edition 1 and edition 2, DNS
edition 1 and 2 and so on. This might have an
effect of stimulating interest in the new bookonly
edition and making available general information
about products that can be useful although has
been superceded by the new, book only edition.
This approach seems particularly suitable when
the newest edition is largely expanded; such
as the second editions of Sendmail, Les & Yacc
or CGI Programming on the World Wide Web.

Your efforts in making your out of print books
are commendable; you might also consider a
similar effort with older, not readily availble
books and old editions.