SharePoint Online - Linking excel sheets to one Master

A user has a new request for Office 365 / SharePoint. 

They want a simple platform for storing reports. They want to have 18 different document libraries, each with different access settings for privacy. Each document library will contain bi-weekly updates, and they want each of these documents to feed into one master spreadsheet, and every time there's an update it syncs to the master. Has anyone done this before? I googled and saw a lot of old posts without a clear solution.

Thanks!

Martha