Is it considered backbiting to say: "This book is better because it is easy, and the other is not good," or "This author's style is better than that author's"? And is criticizing the errors found in a book - for the purpose of benefiting - considered backbiting, whether the author is Muslim or not?

What was mentioned is not backbiting; rather, it is advice, which is the foundation of religion. The one consulted is entrusted and should not betray the one seeking counsel by concealing what is beneficial. And whoever advises his brother on a matter, knowing that rectitude lies elsewhere, has betrayed him. Therefore, a Muslim must advise his brother and clarify to him the truth of what he knows. However, it is not permissible for him to mention faults of authors that are unrelated to their books, or to mention faults that are not in them, for that is backbiting or slander.