Is "AoE - the X Chapters" an extra addition to "AoE 3rd Edition" because everything could not be fitted into one book? Or is it something fundamentally differen

Is "AoE - the X Chapters" an extra addition to "AoE 3rd Edition" because everything could not be fitted into one book? Or is it something fundamentally different? That would make it a set of 3 books to have viz. 1) AoE - 3rd edition 2) X-Chapters 3) Learning AoE. Have i understood correctly? PS: Does the 3rd edition omit a lot from the 2nd edition? What are the major differences?