"It is the man's book."

You'd only use 'det/den' when your replacing a noun you've previously referred to. Given the context we wouldn't know whether or not this book has been referred to, but it most likely wasn't:

'Den er boken til mannen' would mean 'Den boken er boken til mannen', which is redundant. It could be said if someone asked whose book it was and you needed to be specific of which book you were talking about when you answered. Most often you would still answer with the above sentence. I've accepted 'den' in this case, but it wouldn't be very common, and only when you needed to specify specifically what "book" you were talking about.

There are two ways to phrase possession, and each varies slightly when a possessive adjective is used.

The first is when the possessor goes first. Note that the possessee is indefinite.

Det er prestens bord. That is the priest's table. Det er mitt bord. That is my table.

The second is when it comes second. Note that the possessee is definite.

Det er bordet til presten. That is the priest's table. Det er borden mitt. That is my table.

They're interchangeable, with slightly different tones and dialect preferences.

The <ø> is for plurals:

book = bok

the book = boken

books = bøker

the books = bøkene