Hi
Arbortext puts out a FOSI reference manual that you can access from the support Web site and it is ok, especially if you already have a FOSI to work with.
If you are currently suing SGML then stay with it, but if you have a choice go with XML which is a derivative of SGML and does not have to be compiled to use like the SGML DTDs in Arbortext need to be.
If you do not already have FOSIs then I would chose Styler going forward, it is actively supported, allow for creation of both PDF and HTML outputs, actively supported in APP, the FOSI Publisher engine is going on sustainment meaning no new updates as of version 7 I believe. There are on-line classes for learning Styler from Arbortext University.
Ray