ISKCON devotees writing and Srila Prabhupada
Transcendental literature that strictly follows the Vedic principles and the conclusion of the Puranas and pancaratrika-vidhi can be written only by a pure devotee. It is not possible for a common man to write books on bhakti, for his writings will not be effective.
He may be a very great scholar and may be expert in presenting literature in flowery language, but this is not at all helpful in understanding transcendental literature. Even if transcendental literature is written in faulty language, it is acceptable if it is written by a devotee, whereas so-called transcendental literature written by a mundane scholar, even if it is a very highly polished literary presentation, cannot be accepted.
The secret in a devotee's writing is that when he writes about the pastimes of the Lord, the Lord helps him; he does not write alone. As stated in the Bhagavad-gita (10.10), dadami buddhi-yogam tam yena mam upayanti te. Since a devotee writes in service to the Lord, the Lord from within gives him so much intelligence that he sits down near the Lord and goes on writing books.