Please find the up-to-date information on SWI-Prolog at http://www.swi-prolog.org.
Documentation is available from several locations and in several formats.
Paper format can be ordered as print on demand and is available as PDF from the download page.
Several tutorials can be accessed from the Tutorials menu on the home page
A HTML version of the documentation is in the doc/Manual directory
of the installation. Note that some packagers put this documentation
elsewhere or require it to be installed seperately.
You can also install the website locally to use its richness if you are offline. It is available from https://github.com/SWI-Prolog/plweb.