Easily and Free is online so many sources too many to list but a google search will show many.
As far as learning fastly, this all comes down to how well you are at learning content. If you need visual video how to's etc, Youtube may have some content on this. Otherwise there is all sorts of info out there on google as well as local library might even have books on learning DOS and even better Batch that goes hand in hand with DOS.
*Only drawback to old books on DOS is that DOS has evolved over the years and so if the book is talking about DOS 2.11 there is a lot of information missing from Modern DOS Shell as well as some features of DOS 2.11 no longer exist from DOS 2.11 from around 1984. Systems with Windows OS also have VBScript which came with every version of Windows since Windows 98.
