Building applications using multiple datasources for single objects

I'm struggling finding a right solution for my application architecture. For my application I have a single class for customers. The data for filling my customer objects are spread over multiple different types of datasources. The main part is exposed in a readonly Oracle database, other parts are exposed using a webservices and I need te save some extra data to another datasource (for instance a MS SQL database using entityframework) since I only have readonly rights for most datasouces (they are managed somewhere else).

For this reason I wanna build some kind of central library with connectors to all of my datasources for creating a centralized Customer Object to work with. So far so good for this idea (I think) but I can't find any documentation or example with best practices how to achieve such a solution.

EXAMPLE:
  * Main Application (multiple applications)
     - Central Business Logic Layer (Business-API)
         * Webservice Connector
         * Oracle Connector
         * EntityFramework Connector

Does anyone know if there is some good reading material on this specific subject?

Kind regards