PMDG offers some of their manuals (737NG and 744) for download at:
http://www.precisionmanuals.com/html/downloads/docs.htm
Both manuals include a quite detailed description of the FMS. Apart from that, their simulations make learning FMS usage fun and the 737 even runs on my laptop (with the ancient ATI Radeon). Makes a good training environment not only for FMS, but for some aspects of flight and system management.
The 744 Type Rating courses are worth a look, too.
Regards
Cole