The Compleat Atlas of the House and Immediate Environs is a slim, green notebook created by The Architect. It materializes following Arthur's first encounter with Mister Monday. Though it is a portable size when shut, it can resize itself when it is open, often becoming far larger.
The Atlas not only provides maps of the House but also gives information about its features and inhabitants. When Arthur is holding a Key (or when the residue of a Key's power is still in him) he can concentrate on a question and the Atlas will provide the answer. When The Architect has the Atlas on his/her body, it automatically fills his mind with all the information stored within it.
The Seventh Key had the power to block use of the Atlas, even when Arthur is holding the Fifth and Sixth Keys, and has residue from the remaining keys.
The Atlas is much a like a Key; It cannot be taken from the Rightful Wielder , unless they give permission for it to be taken.
It may not often open without a key or residue from a key and will appear as if all the pages are stuck together.