My standard response to this question is to first purchase all the loading manuals you can afford. Each manual comes to its data from a slightly different direction, some manuals have larger selections of powders listed, and some manuals have bullet specific data. I have also come across enough mistakes in the published data that I like to check one source against another when I am trying a new combination. All of the manuals give pretty good general instructions concerning how to actually load ammunition, but depending on your area of interest, some manuals are better for hints when loading hunting ammo, match ammo, service rifle ammo, long range ammo, or handgun ammo.