It can be "the man his book or the man's book", meaning the same, so why is it not accepted
"The man his book" is not good English, but you could translate it as any of the following:
"The man's book"
"The book of the man"
"The book belonging to the man"
I got corrected for missing the apostrophe in "man's", but it said "you are missing a space" and suggested "man_s". I think there may be a minor bug.
Bøker is the indefinite plural form of the noun, so you'd have to put the possessive before it:
mannens bøker ("the man's books").
When you use the definite plural form, you put the possessive after it:
bøkene til mannen ("the books belonging to the man").
Edit: Replaced "o" with "ø" to reflect
bok's vowel change.