Does "boken" here can be her/his book?
No. Then it would be "hans/hennes bok."
"Boken" simply means "the book."
Oops, I got the answer long time ago. However, thanks for the opinion.
As the author creates the world in the book. It means he/she also creates the book.
So in this case "boken" here is "sin bok", not "hennes/hans bok" (another person's book).
Would it sound strange in Swedish to say - Författaren skapar hans egen värld i boken.
Change "hans" to "sin" and it's fine.
Hans/hennes/deras vs. sin/sitt/sina is a matter of reflexive possessive pronouns. See my comment to Caian_Caramori here: https://www.duolingo.com/comment/5640778
ah thanks to you both - I keep forgetting about sin!
I'm glad that version would sound ok in swedish as it more closely lines up with english.
Yes. The construction with hans implies it's someoen else's world in the book in addition to sounding a little bit awkward. To refer back to the author, you'd have to use sin.
How would you say "a world of its own"? Meaning that the author creates a world in the book, not necessarily "her world".
It does not sound strange, it sounds good. However, I would suggest you change "hans" to "sin."