Lets say there is a table named fictional_characters, that looks like:

id          id_novel               character_name
1          1100                        Harry
2          1100                        Ron
3          1100                        Hermione

4          1101                        Frodo
5          1101                        Sam

SELECT  id_novel,
GROUP_CONCAT(character_name)
FROM  fictional_characters
GROUP BY id_novel;

Check the complete reference of GROUP_CONCAT at:

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat