Sword provides an interface to different modules (Bibles/Commentaries/Lexicons) on disk. The object to work directly with the modules is SWModule. Use the class SWMgr to manage the modules.
If you want to write your own frontend for Sword please have a look at the already existing ones. Well knwon frontends are:
-BibleCS for Windows (the sourcecode is availble in the CVS of crosswire.org)
-GnomeSword (http://gnomesword.sourceforge.net/)
-BibleTime (http://www.bibletime.de/)