The functionality of an ereader module that is displaying an electronic book (“ebook”) is extended. A user identifier specifying a user using a client device executing the ereader module and an ebook identifier specifying the ebook displayed by the ereader module are received from the client device. One or more layers associated with the user are determined. One or more layers associated with the ebook are determined. A list of one or more layers is assembled. The layer list is transmitted to the client device. The client device enables the user to select a layer and use the selected layer to interact with content of the ebook.