14. A non-transitory computer-readable medium encoded with a computer program comprising instructions that when executed operate to cause a computer to perform operations comprising: identifying image features in images from a plurality of distinct content items; determining that, for a subset of the distinct content items, the content items in the subset each have respective images that have at least a minimum number of the identified image features in common; generating implicit links between the content items in the identified subset based on determining that the content items in the subset have at least the minimum number of the identified image features in common; assigning weightings to the implicit links between the subset of the content items; and determining a rank score for at least one of the content items in the subset based at least in part on the weightings of the implicit links, the rank score being a value indicative of the importance of the content item relative to others of the distinct content items.
14. A non-transitory computer-readable medium encoded with a computer program comprising instructions that when executed operate to cause a computer to perform operations comprising: identifying image features in images from a plurality of distinct content items; determining that, for a subset of the distinct content items, the content items in the subset each have respective images that have at least a minimum number of the identified image features in common; generating implicit links between the content items in the identified subset based on determining that the content items in the subset have at least the minimum number of the identified image features in common; assigning weightings to the implicit links between the subset of the content items; and determining a rank score for at least one of the content items in the subset based at least in part on the weightings of the implicit links, the rank score being a value indicative of the importance of the content item relative to others of the distinct content items. 17. The non-transitory computer-readable medium of claim 14 , wherein each of the distinct content items corresponds to a published book.