SWI-Prolog reference manual
SWI-Prolog C-library Constraint Query Language A high level interface to SQL databases Pengines: Web Logic Programming Made Easy A C++ interface to SWI-Prolog SWI-Prolog RDF parser SWI-Prolog Semantic Web Library 3.0 SWI-Prolog SSL Interface Managing external tables for SWI-Prolog Transparent Inter-Process Communications (TIPC) libraries SWI-Prolog Unicode library
A paperback version of the manual is available.