well, you couldn't say "une vedette de la musique pop Britney Spears a écrit trois livres". You really need comas. Without them it would make no sense to speak about any famous pop singer and to name her at he same time. But with comas this becomes possible, and the sentence may take a different meaning, based on which part comes first (the first part is then the subject, the other part gives a precision on this subject if it lacks any).

Here are correct sentences in french, I hope I can explain their differences in english :

1) "la vedette de la musique pop Britney Spears a écrit trois livres."

= Famous pop singer Britney Spears has written 3 books.

2) "la vedette de la musique pop, Britney Spears, a écrit trois livres."

= The unique famous pop singer (aka Britney Spears) has written 3 books.

3) "Britney Spears, une vedette de la musique pop, a écrit trois livres."

= Britney Spears (PS : she's a famous pop singer) has written 3 books.

4) "une vedette de la musique pop, Britney Spears, a écrit trois livres."

= There's one famous pop singer who has written 3 books (PS : I'm speaking of Britney Spears).