Since I'm new to shooting, I'm trying to learn as much as possible. Recently I went shopping for ammo and walked out absolutely confused. I ended up buying some Federal Hydra for my XD 9SC, it seemed like a lot of people here use it. When looking at the ammo I realize there is a difference between 115 grain, 124 etc., but what is it? What does that all mean? Next, it seems like +P is pretty popular for home defense in 9mm as well as other calibers, what's the forums opinion of that? Finally, are there any good books on ammunition and the hobby in general that I should be reading? What would you consider MANDATORY reading for a newbie? Thanks.