When Srila Prabhupada has written so many books why do devotees need to write any further books?

Relevant quotes of Srila Prabhupada:

“All students should be encouraged to write some article after reading Shrimad-Bhagavatam, Bhagavad-gita, and Teachings of Lord Caitanya. They should realize the information, and they must present their assimilation in their own words. Otherwise, how they can be preachers?” (letter to Brahmananda, July 1, 1969)

“I want all our students to write articles for our transcendental magazine.” (letter to Satsvarupa, January 11, 1971)

“Regarding articles for BTG, I have already issued instructions to all centers requesting my disciples to send articles every month, and I am going to repeat it again for the second time. ” (letter to Hayagriva, July 12, 1969)

“BTG should be full with our own articles, and there is no need for articles by outsiders.” (letter to Brahmananda, July 1, 1969)

“Realization means you should write. Every one of you. What is your realization? What for this BACK TO GODHEAD is? You write your realization—what you have realized about Krishna. That is required. It is not passive. Always you should be active. Whenever you find time, write. Never mind—two lines, four lines, but you write your realization. Shravanam kirtanam. Writing or offering prayers, glories—this is one of the functions of the Vaishnava. You are hearing, but you have to write also. Then, writing means smaranam—remembering what you have heard from your spiritual master. Shravanam kirtanam vishnoh. About Vishnu, not for others. Don’t write any nonsense thing for any nonsense man. Useless waste of time. Vishnu—write about Vishnu, Krishna. This is cultivation of Krishna consciousness. Hear, write, remember, try to understand. Don’t be dull—dull-headed. Very intelligent. Without being very intelligent nobody can have full Krishna consciousness. It is for the most intelligent man. So that intelligence will come if you try to understand Krishna. We have got so many books. Always try to understand Krishna. Then you are liberated …. You cannot understand Krishna in full—He’s unlimited, our knowledge is limited. But as far as you can, try to understand Krishna.” (lecture, Los Angeles, 1970)

Instructions from Caitanya-caritämåta

avaishnava-mukhodgirnam putam hari-kathamritam
shravanam naiva kartavyam sarpocchishtam yatha payah

Transcendental literature that strictly follows the Vedic principles and the conclusion of the Puranas and Pancaratrika-vidhi can be written only 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 expert in presenting literature in flowery language, but that 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 himself. As stated in 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.” (Adi-lila, Volume 2, pages 189-190)

“To write about the transcendental pastimes of the Supreme Personality of Godhead is not an ordinary endeavor. Unless one is empowered by the higher authorities, or advanced devotees, one cannot write transcendental literature, for all such literature must be above suspicion, or, in other words, it must have none of the defects of conditioned souls, namely, mistakes, illusions, cheating, and imperfect sense perceptions. The words of Krishna and the disciplic succession that carries the orders of Krishna are actually authoritative. To be empowered to write transcendental literature is a privilege in which a writer can take great pride. As a humble Vaishnava, Krishnadasa Kaviraja Gosvami, being thus empowered, felt very much ashamed that it was he who was to narrate the pastimes of Lord Caitanya Mahaprabhu.” (Adi 2, page 206)

“Anyone who attempts to write about Krishna must first take permission from the spiritual master and Krishna. Krishna is situated in everyone’s heart, and the spiritual master is His direct representative. Thus Kirsna is situated antar-bahih, within and without. One must first become a pure devotee by following the strict regulative principles and chanting sixteen rounds daily, and when one thinks that he is actually on the Vaishnava platform, he must then take permission from the spiritual master, and that permission must also be confirmed by Krishna from within his heart. Then, if one is very sincere and pure, he can write transcendental literature, either prose or poetry.” (Adi 2, pages 206-207)

“A sentimentalist who has no Vaishnava qualifications cannot produce transcendental writings. There are many fools who consider krishna-lila to be a subject of art and write or paint pictures about the pastimes of Lord Krishna with the gopis, sometimes depicting them in a manner practically obscene. These fools take pleasure in material sense gratification, but one who wants to make advancement in spiritual life must scrupulously avoid their literature. Unless one is a servant of Krishna and the Vaishnavas, as Krishnadasa Kaviraja Gosvami presents himself to be in offering respects to Lord Caitanya, His associates, and His disciples, one should not attempt to write transcendental literature.” (Adi 2, page 215)

“This is the sum and substance of transcendental writing. One must be an authorized Vaishnava, humble and pure. One should write transcendental literature to purify oneself, not for credit. By writing about the pastimes of the Lord, one associates with the Lord directly. One should not ambitiously think, ‘I shall become a great author. I shall be celebrated as a writer. These are material desires. One should attempt to write for self-purification. It may be published, or it may not be published, but that does not matter. If one is actually sincere in writing, all his ambitions will be fulfilled. Whether one is known as a great author is incidental. One should not attempt to write transcendental literature for material name and fame.” (Adi 2, page 216)

(From the Back to Godhead Handbook)