But the sentence is very strange. It sounds a little like something a German teacher of English might have invented; even the word order 'written beautifully' is a little unusual.
Off the top of my head, I can't think of any context in which it would be very idiomatic, because it combines two kinds of descriptions: a general description of an inherent characteristic, such as the identity of the writer or the style of writing, vs. a narration of a particular event, such as the composition of a book or the physical writing of a manuscript or other historic document.
Compare the verb tenses in these examples:
The book is beautifully written, in a poetic style. It is by Virginia Woolf.
She looked back down at the novel lying open on her desk. It was beautifully written, by a woman writer called Virginia Woolf. She had never heard of her.
The novel was written in the summer of 1888, when Brontë was staying with a friend in London.
He thought about the other Brontë novel. It was so beautifully written, yet so sad. It had been written in the summer of 1888, before Brontë returned to Yorkshire.
The manuscript is beautifully written, in a hand typical of a 16th-century monk.
The detective examined the will. It was beautifully written, in Peters's typical flowing handwriting, but there was something odd about the signature.
The detective examined the will. It was signed "John Peters."
The historian examined the book. It appeared to have been written by a man called John Peters, but the style was surprisingly elegant for a no-name author.
Do you see now why 'beautifully written,' 'had been written,' and 'by a man called John Peters' don't belong in the same sentence?
Another way of thinking about it: When you use 'is/was beautifully written,' the participle 'written' is simply like an adjective, a general description. (I think in German this would be 'ist/war geschrieben.' But 'had been' is a little strange here, just as I imagine it might be strange to say 'war wunderschön geschrieben gewesen.')
Usually, when you use 'had been written,' the implication is that it's the true passive voice, narrating an action performed by a person at a particular time. (I think in German this would be 'wird/wurde geschrieben/war geschrieben worden.')