If you are reusing content (such as quoting from a book or journal, or reproducing an image) from a work that is still in copyright, you most likely need to seek permission from the rightsholder, who may be the publisher, author or author’s estate.
There are certain types of reuse that do not require you to seek permission, such as use that falls under ‘fair dealing’. More information on exceptions to copyright and fair dealing can be found here: www.gov.uk/guidance/exceptions-to-copyright.
For reuse that does not fall under any copyright exceptions and for a work that is still in copyright, permission will need to be sought.