substitute for The Black Arrow?

I am new to CM and my 16-yr-old son is bored with “The Black Arrow” by Stevenson(suggested in Mid. Ages, Ren, etc.) Should I tell him to stick with it or provide an alternative? If yes, what book do you suggest?

I am having a hard time figuring out which books are spines and/or essential. Like many I fear my uneducated choices will short-change him.

I need ideas for next year’s essential books too for vol. 5.

If I could afford it, I would just buy all the suggested books:)

Thanks!