1. A method for mapping supplemental content layers to a source document by an electronic publishing platform, the source document being a later edition of a target document, the method comprising: accessing the target document, the target document having a supplemental content layer mapped to respective content items in the target document, the target document being an earlier edition of the source document; generating, by the electronic publishing platform, indexes mapping content items of the source document to the content items of the target document based on a correlation between the source document content items and the target document content items; and mapping the supplemental content layer to the source document based on the indexes.