Interesting. Personally, I feel like there's a slight difference in emphasis between "it is not his book" and "the book is not his", which seems to be perfectly captured by the Romanian sentences "nu e cartea lui" and "cartea nu e a lui".
I'm not sure how to express it, but I feel like the first sentence focuses on him, while the second focuses on the book. Maybe it's just me.
this is something duolingo doesn't teach very well. if you are describing ownership of an item, the noun is given the definite article.
pisica mea, caînele tau, intrebarile voastre.
in English, I would literally say this as "the dog of you". in practice, this means the same as "your dog".
You could also say "It is not the book of his.", but it is not as colloquial as "It is not his book."
In English the first sentence would have to be "It is not the book of him." But that construction would not actually be used. It would just be a word-for-word translation.