Answer:
It holds most knowledge that has ever been said. But is not the brain, is not the head. To feathers and their masters, it's both bane and boon One empty, and one full.
My first master has four legs, my second master has two. My first I serve in life, my second I serve in death. Tough I am, yet soft beside. Against ladies cheeks I often reside.
I am whole but incomplete. I have no eyes, yet I see. You can see, and see right through me. My largest part is one fourth of what I once was.
Something wholly unreal, yet seems real to I. Think my friend, tell me where does it lie?
The more of it there is, the less you see.
If it’s information you seek, come and see me. If it’s pairs of letters you need, I have consecutively three. What am I?