16. A system, comprising: a data processing apparatus; and a data store storing instructions that, when executed by the data processing apparatus, cause the data processing apparatus to perform operations comprising: identifying images that appear in a plurality of distinct book content items, the distinct book content items corresponding to published books and lacking explicit electronic links between each other; generating implicit links between two or more of the distinct book content items that each include a similar image, the links represented as weighted edges in a graph in which nodes represent corresponding book content items, each weighted edge representing one or more matches of image content in corresponding book content items as between different books for nodes that define a corresponding weighted edge; for particular distinct ones of the nodes, identifying images that match each other as between different book content items based on multiple descriptor points for each of the images, and assigning weightings to particular edges between the distinct ones of the nodes; and determining a rank score for each of the two or more distinct book content items based on the implicit links between the two or more distinct book content items, the rank score for each of the two or more distinct book content items being a value indicative of the importance of the distinct book content item relative to other distinct book content items.
16. A system, comprising: a data processing apparatus; and a data store storing instructions that, when executed by the data processing apparatus, cause the data processing apparatus to perform operations comprising: identifying images that appear in a plurality of distinct book content items, the distinct book content items corresponding to published books and lacking explicit electronic links between each other; generating implicit links between two or more of the distinct book content items that each include a similar image, the links represented as weighted edges in a graph in which nodes represent corresponding book content items, each weighted edge representing one or more matches of image content in corresponding book content items as between different books for nodes that define a corresponding weighted edge; for particular distinct ones of the nodes, identifying images that match each other as between different book content items based on multiple descriptor points for each of the images, and assigning weightings to particular edges between the distinct ones of the nodes; and determining a rank score for each of the two or more distinct book content items based on the implicit links between the two or more distinct book content items, the rank score for each of the two or more distinct book content items being a value indicative of the importance of the distinct book content item relative to other distinct book content items. 23. The system of claim 16 , wherein determining a rank score for each of the two or more distinct book content items comprises: representing the two or more distinct book content items as a graph in computer memory, where each of the distinct book content items is represented as a distinct node in the graph and where an edge exists in the graph between each pair of distinct nodes that represent distinct book content items that both include a similar image; and determining a rank score for each of the two or more distinct book content items based on the edges between the distinct nodes.