select title,category,price from books1 where books1.category='MYS';
I'm not sure why you have books1 when the model show the table named books. Books is a horrible name for a table -- typically in relational databases you use the singular -- eg book.
Here is how you do a join. I hate to break it to you but this is very basic sql -- if this is a test on databases you are not going to do well on your final tomorrow.
Select a.First, a.Last from books b join books_authors ab on b.b_code = ab.book_code join authors a on ab.authorId = a.id where b.category = 'MYS'
also all of your field names have spaces in them -- I don't know what platform you are using so I've no idea how to escape the names. Using spaces in field name is non-standard and not acually SQL. I'd advise against it whenever possible.