It’s bizarre that it’s so hard. But it is not at all clear how to read Marvel’s mid-nineties series the age of Apocalypse. Sure, there are references, like here. But even these presume some background, and mix in information for collectors. If you simply want to read Age of Apocalypse, here is how to do it:
Book 1 makes absolutely no sense. It doesn’t contain any narrative. Skip it, or read it after 4.