how to retrieve refrencing parent id with EntityFieldQuery I am very new to EntityFieldQuery. I understand how you create a query with this class. However, I have a node which is referenced (with Entity Reference) by a parent node. I would like to use EntityFieldQuery to retrieve the parent id. Can this be done?
Without extending the class I'm pretty sure not. The reverse relationship isn't defined as a field, so EntityFieldQuery won't be able to help. 

Don't forget entity reference doesn't define a parent/child relationship, it's literally just a relationship from one entity to another. A single entity can be referenced by any number of other entities, so the notion of a single 'parent' doesn't conceptually exist. 

If you want more advanced relationships you might consider converting to the Relation module. It has a RelationQuery class which wraps around EntityFieldQuery to add relationship-specific functionality.