You don't need to get any books to build an army for
AoS really, however some have battletomes which add things like spells and whatever (only the sylvaneth, bonesplittaz and soon to be released beastclaw at the moment have this, the rest will be just narrative missions, background and the warscrolls for the units you can get for free anyway)
The
GHB is where all the points are though as well as the rules for matched play using it. The rules for everything else you'll basically be tripping over everywhere anyway.