14. The system of claim 13 , wherein each distinct book content item is an electronic representation of a distinct book.
13. A system, comprising: a memory, the memory storing an index of distinct book content items and corresponding characteristics of the distinct book content items; one or more computers configured to interact with the memory, the one or more computers being further configured to perform operations comprising: identifying image content for a plurality of distinct book content items, the image content defining images that appear in the plurality of distinct book content items; identifying descriptor points from the image content, the descriptor points defining localized features of the image content of the distinct book content items, the localized features being characteristics of a portion of the image content; representing the distinct book content items as a weighted graph in computer memory, where each of the distinct book content items is represented as a distinct node in the weighted graph and where an edge exists in the weighted graph between each pair of distinct nodes that represent distinct book content items that both include a matching descriptor point, matching descriptor points being identified based on similarities between pairs of descriptor points, and each edge in the weighted graph being weighted based on a relative importance of the distinct node from which the edge originates; for each distinct node: identifying matching image content in the image content of other distinct book content items that are represented by other distinct nodes, each matching image content being image content that has descriptor points that match the corresponding descriptor points of image content in the distinct book content item corresponding to the distinct node; generating edges in the weighted graph connecting the distinct node to other distinct nodes corresponding to the other distinct book content items, each edge representing one or more matches of image content in the distinct book content item to matching image content in another distinct book content item; determining a rank score for each distinct book content item based on the edges between the distinct nodes representing the distinct book content items, the rank score being a score indicative of the importance of each distinct book content item relative to other distinct book content items.