There are not many textbooks that specialize in the study of writing, but there are several. And I recommend the following textbooks.
「10の基本ルールで学ぶ 外国人のためのビジネス文書の書き方」
「タスクで学ぶ日本語ビジネスメール・ビジネス文書」
「日本語ビジネス文書マニュアル」
「日本語でビジネスメール ―書き方の基本と実用例文」
「しごとの日本語 メールの書き方編」

I was able to ask Shirasaki-san, the author of the topmost textbook, about the key to developing business writing skills, so let me share her comments with you.
—————————————————————–
Many people have problems with business documents, such as “I don’t know what to write and how to write it,” or “It takes too much time to write. There are three key points to developing your business document writing skills.

(1) Master the “expressions” of business Japanese.
The most important aspects of “expression” in business documents are “accuracy” and “conciseness. To achieve this, please pay attention to the following four points.
(a) Written language, not spoken language
(b) Honorifics
(c) Headings
(d) Bullet points
If you find these difficult, meeting minutes or event invitations written by others may be helpful.
 
(2) Master the “formats” of business Japanese.
There are general “formats” of business documents. The “formats” differs between documents for external and internal use. Some companies have detailed rules for layout and items to be written.
If you find these difficult, documents written by your seniors or internal document rule books (if available at your company) may be helpful.

(3) Check from the reader’s perspective.
Be sure to read over the business documents you have created. If it is difficult to have a Japanese colleague or senior colleague check it, try using the following checklist.
☑ Is the “conclusion” you want to convey to the other party properly written?
☑ Are the items in the correct order and are there any omissions?
☑ Are the sentences short and simple?
☑ Are there any errors in proper nouns, numbers, or notations?
☑ Check whether it is intended for internal or external use, and whether it is written in an appropriate style.
—————————————————————–

In Shirasaki’s book, you can learn the 10 rules that foreign learners of Japanese are likely to make mistakes (such as stylistic unification), and then learn how to structure “daily reports,” “training reports,” “minutes,” “request for approval,” and “proposals,” which are frequently prepared by businesspersons. If you are interested, please check it out. https://amzn.to/3Lqgqxe

Social Media