Answer:
I'm white, I'm round, but not always around. Sometimes you see me, sometimes you don't.
My first is in wield, sever bones and marrow. My second is in blade, forged in cold steel. My third is in arbalest, and also in arrows. My fourth is in power, plunged through a shield. My fifth is in honor, and also in vows My last will put an end to it all.
What can be measured but not seen?
This is needed both for courage and hardcover books.
What has two spines and a lot of ribs, and carries much but never moves?
What is always coming but never arrives?