11. A computer-readable medium encoded with a computer program comprising instructions that, when executed, operate to cause a computer to perform operations comprising: receiving in a computer system a search query; determining, for each web page in a plurality of web pages that satisfy a search query and have a position in a ranked order of web pages that satisfy the search query, whether the web page is a reference page for a book, wherein a particular web page is determined to be a reference page for a particular book when (i) the web page includes a reference to the particular book and has a citation score that satisfies a citation criterion for the particular book, (ii) a citation score of the web page for the particular book exceeds a citation threshold, and (iii) citation scores of the web page for other books referenced on the web page are each less than the citation score for the particular book, wherein the citation score is a measure of relevance of book metadata for the particular book to content of the web page; selecting, from among the web pages, a subset of web pages that are each a reference page for a respective book; assigning a book score to each of the books for which there is at least one reference page in the subset of web pages, where each of the reference pages in the subset referencing a respective book contributes to the book score for the respective book; selecting one or more of the books based on the book scores; generating a book reference for each of the one or more books, wherein each book reference includes at least one of citation information of the respective book or a link to a page of the respective book; and providing, in a response to the search query, the book references in addition to one or more web content references, each web content reference linking to one of the plurality of web pages.
11. A computer-readable medium encoded with a computer program comprising instructions that, when executed, operate to cause a computer to perform operations comprising: receiving in a computer system a search query; determining, for each web page in a plurality of web pages that satisfy a search query and have a position in a ranked order of web pages that satisfy the search query, whether the web page is a reference page for a book, wherein a particular web page is determined to be a reference page for a particular book when (i) the web page includes a reference to the particular book and has a citation score that satisfies a citation criterion for the particular book, (ii) a citation score of the web page for the particular book exceeds a citation threshold, and (iii) citation scores of the web page for other books referenced on the web page are each less than the citation score for the particular book, wherein the citation score is a measure of relevance of book metadata for the particular book to content of the web page; selecting, from among the web pages, a subset of web pages that are each a reference page for a respective book; assigning a book score to each of the books for which there is at least one reference page in the subset of web pages, where each of the reference pages in the subset referencing a respective book contributes to the book score for the respective book; selecting one or more of the books based on the book scores; generating a book reference for each of the one or more books, wherein each book reference includes at least one of citation information of the respective book or a link to a page of the respective book; and providing, in a response to the search query, the book references in addition to one or more web content references, each web content reference linking to one of the plurality of web pages. 12. The computer-readable medium of claim 11 , wherein the citation scores of the web page for other books referenced on the web page are each at least a predetermined percentage less than the citation score for the particular book.