There is no tutorial, you can get some tips on working in graphical apps if googled, but you have to printsreen manually every unit in the game (custom battles are best for).

Is there a location where I can view the descriptions of the list of units? I like to make recruitment decisions based on historical data etc. Perhaps you have source material links I could also go to read up on these units. Thanks!

Unfortunately there is no such a place and i don't think it ever come. Historicity of the units is only matter of speculation, they mostly represent some classes of warriors, so division between spear and sword infantry is forced by the engine, most units those times were equipped both in sword and spear. If you look for Notitia's units, you will find good info here:
http://lukeuedasarson.com/NotitiaPatterns.html