der Verfasser
author
A person who has written a written text, such as a book, an article, a letter, or an expert report.
Der Verfasser dieses Romans ist leider unbekannt.
The author of this novel is unfortunately unknown.

author
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author
A person who has written a written text, such as a book, an article, a letter, or an expert report.
Der Verfasser dieses Romans ist leider unbekannt.
The author of this novel is unfortunately unknown.

'Verfasser' (author) is derived directly from the verb 'verfassen' (to write or compose), which means 'to write or create a text.' A 'Verfasser' is therefore the person who has written a text.
Es dauerte Monate, dieses Buch zu verfassen.
It took months to write this book.
The word 'Verfasser' (author) belongs to a rather formal or official register. It is often found in academic, legal, or literary studies texts.
Der Verfasser des Gutachtens wurde zur Anhörung geladen.
The author of the expert report was summoned to the hearing.
Very often, 'Verfasser' (author) is followed by a genitive that indicates which text is meant. Typical formulations are 'der Verfasser des Buches' (the author of the book) or 'die Verfasserin des Artikels' (the author of the article).
Die Identität des Verfassers der anonymen Briefe konnte nie geklärt werden.
The identity of the author of the anonymous letters could never be established.
author
'der Autor' and 'der Verfasser' can both refer to the person who wrote a text. 'Autor' is especially common for books, articles, and published texts. 'Verfasser' sounds somewhat more factual and fits well with letters, reports, expert opinions, or official texts. The words are easy to confuse because they can often refer to the same person. If you are speaking generally about a book or an article, 'Autor' is very natural. If you want to say more precisely who wrote a particular text, especially in a factual context, 'Verfasser' fits well.

Mila hat heute den Verfasser des Berichts angerufen.
Mila called the author of the report today.
Der Autor beantwortet nach der Lesung Fragen aus dem Publikum.
The author answers questions from the audience after the reading.