1. A computer-implemented method for mapping supplemental content layers to an electronic source document stored by an electronic publishing platform, the source document being a later edition of an electronic target document, the method comprising: accessing the target document, the target document having an electronic supplemental content layer including electronic content associated with content of the target document, wherein portions of the electronic content of the supplemental content layer are mapped to respective content items in the target document, and wherein the target document is an earlier edition of the source document; for each of a plurality of content items in the target document: identifying a content item in the source document similar to the content item in the target document; adding, to an index stored by the electronic publishing platform, a mapping between the identified content item in the source document and the content item in the target document; and mapping the portion of the electronic content of the supplemental content layer mapped to the content item in the target document to the identified content item in the source document based on the index; and storing the portions of electronic content of the supplemental content layer mapped to content items in the source document as a supplemental content layer associated with the source document.