1👍
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.