Since you're specifically trying to write—a book, I guess—it may be worthwhile trying to find an app designed for that. A quick search came up with Draft (https://draftin.com/) and Reeday Book Editor (https://reedsy.com/write-a-book). I haven't used these myself, so take a look around. Source: over 1 year ago

Similarly, cloud based Markdown editors such as HedgeDoc, StackEdit, HackMD and Draft among others are not great choices for those who don't have Markdown experience, plus are a bit more barebones than desired for project management. The same applies for LaTex collaboration software like Overleaf. - Source: dev.to / over 2 years ago

Alternatively, you can use an online markdown editor like StackEdit or HackMD. - Source: dev.to / 4 months ago

Use https://stackedit.io/ in the browser :). Source: 6 months ago

Markdown is awesome! But, when writing 1000 words+ articles, I quickly feel the need for a better experience. For years, I’ve used StackEdit — an open-source, in-browser Markdown editor — for editing all kinds of long-format Markdown text. That said, given my recent experience with WYSIWYG editors, I thought I could do something better. - Source: dev.to / 10 months ago

This is especially annoying as when I export from stackedit.io to HTML, then it just cuts off anything which is outside the greyed in code window! Source: 11 months ago

StackEdit[0] pretty much perfected what I needed out of a markdown editor - I just need somewhere to write my tickets/docs that wasn't Github so that I could format it properly while writing. I still use it from time to time [0]: https://stackedit.io/. - Source: Hacker News / 11 months ago