Explain search (Sphinx/Haystack) in simple context?
Firstly, Haystack isn't a search engine, it's a library that provides a Django API to existing search engines like Solr and Whoosh.
That said, your example isn't really a very good one. You wouldn't use a separate search engine to search by ISBN, because your database would already have an index on the Book table which would efficiently do that search. Where a search engine would come in could be in two places. Firstly, you could index some or all of the book's contents to search on: databases are not very good at full-text search, but this is an area where search engines shine. Secondly, you could provide a search against multiple fields - say, author, title, publisher and description - in one go.
Also, search engines provide useful functionality like suggestions, faceting and so on that you won't get from a  database.
Could you explain how search engines like Sphinx, Haystack, etc fit in to a web framework. If you could explain in a way that someone new to web development could understand that would help.
One example use case I made up for this question is a book search feature. Lets say I have a noSQL database that contains book objects, each containing author, title, ISBN, etc.; how does something like Sphinx/Haystack/other search engine fit in with my database to search for a books with a given ISBN?