I recently did a tune up and pulled the side cover to find a engine family nuber on it.
What is this number for and how do I decifer it?

It allows you to look up parts that have the same engine type for different years. For example, my 750 Maxim and the 81-83 Seca's have very similiar/same engine parts that are interchangable - even amonst years.

Helps when trying to find a particular part - you're not limited to your specific model or year.

Is there a book that can decifer the number for me? is there a way to use this number to verify the originality to the bike?I bought it used and am not sure of any of the history on the bike.