There are a number of reasons witches write books.
Most well known are the yellow paperback sensationalist rags of the 60’s-90’s with red tinted photos of sexy naked witches standing on pentagrams. Obviously these authors write for attention and money. Most pagan authors even popular ones do not have these kind of sales and are supported by a day job or their life partners. So why do they write?
1-For credibility so they can establish respect for their tradition, or as teachers. A number of small traditions share introductory material to show their chops.
2-Because they are sought after as teachers and cannot teach all those interested. So in service of those who seek witchcraft they provide education in the form of books.
3-To help witches who don’t/ cannot practice with covens or study in traditions due to circumstance or location so they can practice on their own.
4-A mystery tradition hoping to grow may put out a book to seek out high quality students. Interested readers will follow up by writing letters or otherwise getting in contact with the authors to pursue study.
5-To cast a spell on the world to make an impact upon it. Many witches right for causes that are close to their hearts and their souls’ purposes. These might be books on magic and the environment for example.
Ultimately the main reason a witch writes a book is because they love writing sometimes they love-hate writing, as its a challenge they are passionate about.