SQL Error: Ambiguous Column Name: media_id

That error probably indicates that both tables have a column named media_id. Just put the table name that you want to select the column from and a period in front of the column name.

For example, if the table name was foo, you might have a query that starts like this:

SELECT foo.media_id ...

I had the same problem. Steven Parker was right. Since there are two tables with the column name media_id it does know what table to pull that column from so you have to add Media before media_id in the SELECT statement. It worked for me after adding it.

SELECT Media.media_id, title, category, img, format, year, genre, publisher, isbn 
            FROM Media
            JOIN Genres ON Media.genre_id = Genres.genre_id
            LEFT OUTER JOIN Books ON Media.media_id = Books.media_id 
            WHERE Media.media_id = $id;