"Is this your book"? wrong because of the word order?
Correct. That would be "Er denne din bog?"
Why not den instead Deanne?
That's a difference between "that" (den) and "this" (denne).
So your/yours aren't separate words then? Gender/plurality aside (din/dit/dine), your/yours isn't differentiated?
I believe that's right, yes. Same with "my" versus "mine" and the others. (No difference with "his" because of the 's'.)
I am doing this on a mobile so don't have access to the lessons.
Why not 'den' ?
It would be helpful if the person speaking would brake her reading of the sentence between bog and din so that it is not read as "bogdin".