GDG- Essential Books on Gettysburg

[Dave]

I want to thank everyone for the discussions on Heth, Stuart, et al.  Now,
what essential books would your recommend for the person who, lets say, is
"training" to be a guide?  Part 2, how would you approach the battle to
understand it thoroughly?  Do you treat it like most research, read all the
surveys then work down to the days, then maybe regiments and personalities?

Dave McGowan
(Cousin of Sam)