* interesting: practical books like "How to be an even better manager", Math books, medical books
* classics: Shakespeare, Walter Scott, Goethe, Francis Bacon, Alexander Pope - all of them are in the public domain and can be downloaded as PDF files
* highly cited in scholar papers: authoritative books like Hegel's "Science of logic", Thomas Hobbes' "Leviathan"
* random subject: books from a randomly-selected topic
Google also lists the main subjects for fiction and non-fiction books and some random subjects. It's interesting that Google Book Search defaults to the cover view when you explore the books from a subject and to the list view when you do a normal search.
For ambiguous queries like [logic], you'll be able to restrict the results to an appropriate subject. You can also use the subject: operator to make your search more precise (for example: war subject:"Science fiction"). Google doesn't offer the complete hierarchy of subjects, so a clever autocomplete would be helpful.
