Should "buku ini milikmu" also be accepted as correct?
No. That would be "this book is yours". Different subjects, different predicates.
I got a multiple choice saying "Ini bukuAnda". I guess there should be a space between buku and Anda?
Here is more info about the word order in a noun phrase :
Adjectives, Noun Phrase, Tips & Notes, Addendum.
'ini' , 'itu', Tips & Notes, Addendum.
Anyone who is wondering why "Buku ini kamu" doesn't work: the second of Rick's links is for you.
Bukumu ini? = Is this your book ?
The voice say bukukamu