There are over 520 letters as Nichiren's writings in the standard edition of Showa era. But there are many unauthentic letters in those. Authentic writings are 113 letters. SGI's edition is out of the question. Let's refer to NOPPA's Writings of Nichiren Shonin (University of Hawaii Press). http://www.uhpress.hawaii.edu/books.aspx
But please be careful, because some passages of Nichiren's writings has been changed by Nichiren-shu's original doctrine. If you have doubts about the sentence, please confirm to me. Of course, you have to read at first the five major works that are the most important, especially Kaimoku sho and Kanjin Honzon sho. And you need to know that Nichiren shonin said that his writings before Sado exile were expedients.

AAA: The five major works
AA: 　Important authentic writings
A: 　　Authentic writings
A': 　　Not perfect authentic writings

Non: 　　　A copied letter by other. There is no handwritten letter by Nichiren shonin.
Caution: Non, and there are some problems in its contents as the thoughts of Nichiren shonin.
Warning: A fully forged letter by other.

DOCTRINE 1

1. Shugo Kokka-ron >A
　(Treatise on Protecting the Nation)
2. Sainan Koki Yurai >A
　(The Cause of Misfortunes)
3. Sainan Taiji-sho >A
　(Treatise on the Elimination of Calaminties)
4. Rissho Ankoku-ron >AAA
　(Treatise on Spreading Peace Throughout the Country by Establishing the True Dharma)
5. Ankoku-ron Soejo >A
　(Covering Letter to the "Rissho Ankoku-ron")
6. Ankoku-ron Gokanyurai >A
　(The Reason for Submitting the "Rissho Ankoku-rin")
7. Yadoya Nyudo Sai-gojo >A'
　(second Letter to Lay Priest Yadoya)
8. Ankoku-ron Okugaki >A
　(Postscript to the "Rissho Ankoku-ron)
9. Ko Saimyoji Nyudo Kenzan Gosho >A'
　(Meeting the Late Lay Priest Saimyoji Letter)
10. Kingo-dono Go-henji >A
　(A Reply to Lord Ota Jomyo
11. Ankoku-ron Sojo >A
　(A Letter Requesting the "Rissho Ankoku-ron")
12.Muso Gosho >A
　(Record of a Dream)
13. Kassen Zai-Genzen Gosho >A'
　(A War Right Under your Nose)
14. Ken Rissho-i Sho >Non
　(A Tract Revealing the Gist of the Rissho Ankoku-ron")
15. Shinkoku-o Gosho >A'
　(Sovereigns of Our Divine Land)
16. Senji-sho >AAA
　(Selecting the Right Time)
17. Gonin-jo Go-henji >A
　(Response to Gonin's Letter)
18. Kangyo Hachiman-sho >A'
　(Remonstration with Bodhisattva Hachiman)

DOCTRINE 2

1.Jissho-sho >A'
　(A Treatise on the Ten Chapters of the Great Concentration and Insight)
2.Teradomari Gosho >A
　(A Letter from Teradomari)
3.Hasshu Imoku-sho >A
　(A Treatise on the Differences of the Lotus Sect from eight Other sects)
4.Kaimoku-sho >AAA
　(Open your Eyes to the Lotus Teaching)
5.Toki-dono Go-henji >A
　(A response to Lord Toki)
6.Shingon Shoshu Imoku >A
　(The Differebces between the Lotus Sect and Other Sects such as the True Word Sect)
7.Kanjin Honzon-sho >AAA
　(A Treatise Revealing the spiritual contemplation and the most Venerable One)
8.Kajin Honzon-sho Soejo >A
　(The Covering Letter of the "Kajin Honzon-sho")
9.Kembutsu Mirai-ki >AA
　(A Testmony to the Prediction of the Buddha)
10.Toki-dono Go-henji >A
　(A Response to Lord Toki)
11.Hakii Saburo-dono Go-henji >Non
　(A Response to Lord Hakii Saburo)
12.Shojo Daijo Fumbetsu-sho >A'
　(The Differences between Hinayana and Mahayana Teachings)
13.Gochu Shujo Gosho >Non
　("People in the World" Letter)
14.Hokke Shuyo-sho >AA
　(A Treatise on theEssence of the Lotus Sutra)
15.Rissho Kanjyo > Caution
　(A Treatise on Establishing the Right Way of Meditation)
16.Rissho Kanjo Sojo > Caution
　(The covering Letter to "A Treatise on Establishing the Right Way of Meditation"
17.Misawa sho >Non
　(A Letter to Lord Misawa of Suruga)
18.Shimon butsujo-gi >A
　(Listening to the One Buddha Vehicle Teaching for the First Time)
19.Toki Nyudo-dono Go-henji:Chibyo-sho >A
　(A Response to Lay Priest Lord Toki: Treatise on healing Sickness)
20.Honzon Mondo Sho > Caution
　(Question and Answers on the Honzon)
21.Toki Nyudo-dono Go-henji:Hongon Shukkai-sho >A
　(A Response to Lay Priest Lord Toki: Treatise on Overcoming Illusions of the Triple World by ProvisionalTeachings)
22.Shokyo to Hokkekyo to Nan'i no Koto >A
　(The Difficulty and Easiness in Understanding the Lotus sutra and Other sutras)
23.Sandai Hiho Honjo-ji >Caution
　(The Transmission of the Three Great Secret Dharmas)

DOCTRINE 3

1.Hoon-jyo >AAA
　(Essay on Gratitude)
2.Hoon-jo Somon >Non
　(Covering Letter to the "Hoon-jyo")
3.Ichidai Shogyo Tai-i >A
　(Outline of ALL the Holy Teachings of the Buddha)
4.Kyoki Jikoku-sho >Non
　(Treatise on the Teachings, capacity, Time, and Country)
5.Ken Hobo-sho >A
　(Clarification of Slandering the true Dharma)
6.Nanjyo Hyoe shichiro-dono Gosho >A'
　(A Letter to Lord Nanjyo Hyoe shichiro)
7.Soya Nyudo-dono-gari Gosho >AA
　(A Letterto Lay Priest Lord Soya)
8.Shoshu Mondo-sho >Non
　(Questions and Answers on the Other Schools)
9.Hokke Jodo Mondo-sho >A'
　(Question and Answers on the Lotus and Pure Land Sects)
10.Soya Nyudo-dono Gosho >A'
　(A Letter to Lay Priest Lord Soya)
11.Ota-dono-gariGosho >A
　(A letter to Lord Ota)
12.San Sanzo Kiu no Koto >A
　(Concerning the Prayer Services for Rain by Three Tripitaka masuers)
13.Daigaku Saburo-dono Gosho >A
　(A Letter to lord Daigaku Saburo-dono)
14.Jikaku Daishi no Koto >A
　(Concerning Grand Master Jikaku)
15.Nizen Nijo Bosatsu Fusabutu Ji >Non
　(Never-Attaining Buddhahood by the Two Vehicles)
16.Nijo Sabutu Ji >Non
　(Obtaining Buddhahood by the Two Vehicles)
17.Ichidai Goji Keizu >A
　(Genealogical Chart of the Buddha's Lifetime Teachings)

FATH AND PRACTICE 4

1. Sho Hokke Daimokusho >Non
　(Treatise on Chanting the Daimoku of the Lotus Sutra)
2. Yakuo-bon Tokui-sho >A'
　(The Essence of the "Medicine King Bodhisattva" Chapter)
3. Hokke Daimoku-sho >A'
　(Treatise on Daimoku of the Lotus Sutra)
4. Zemmui-sho >A'
　(Treatise on Subhakarasimha)
5. Kito-sho >A
　(Treatise on Prayers)
6. Shoho Jisso-sho >Caution
　(Treatise on All phenomena as the Ultimate Reality)
7. Nyosetsu Shugyo-sho >Non
　(The Way of Practicing the Teaching of the Buddha)
8. Mokue Nizo Kaigen no Koto >A
　(Opening the Eyes of Buddist Images, Wooden Statues or Prtraits)
9. Hokyo Hoju Ji >A
　(Dharma Is More Precious Than Treasure)
10. Jiri Kuyo Gosho >A
　(Phenomenal Offering and Noumenal Offering)
11. Shishin Gohon-sho >AA
　(The Four Depths of Faith and Five Stages of Practice)
12. Ueno-dono Gohenji >A'
　(A Reply to Lord Ueno)
13. Toki-dono Gosho >A
　(A Reply to Lord Toki)
14. Shushun Tenno Gosho >A
　(The "Emperor Sushun" Letter)
15. Ueno-dono Gohenji >Non
　(A Reply to Lord Ueno)
16. Ueno-dono Gohenji >Non
　(A Reply to Lord Ueno)
17. Dannotsu-bo Gohenji >A
　(A Response to Followers)
18. Nichinyo Gozen Gohenji >A'
　(A Response to My Lady Nichinyo)
19. Myoho-ama Gozen Gohenji >A'
　(A Reply to My Lady, the Nun Myoho)
20. Sennichi-ama Gozen Gohenji >A
　(A Reply to my Land Nun Sennichi)
21. Kurotaro-dono Gohenji >A'
　(A Reply to Lord Kurotaro)
22. Zui-jii Gosho >A'
　(The Sutra Preached in Acordance to the Buddha's Own Mind)
23. Ueno-dono Gohenji >A
　(A Reply to Lord Ueno)
24. Ota-dono Nyobo Gohenji >A
　(A Response to the ife of Lord Ota)
25. Urabon Gosho >A
　(On the Ullambana Service)
26. Uenodono Haha-ama Gozen Gohenji >A'
　(A response to My Lady the Nun, Mother of Lord Ueno)
27. Hokke Shomyo-sho >A
　(Treatise on Testimony of the Lotus Sutra)

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