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

Nice and simple. I feel the only lacking feature for a basic blog is having unlisted blog posts, which is very handy when you want to share it to proof-readers. This can be done on google doc/hedgedoc [0] for sure, but then when porting there are very often typos creeping in. [0] https://hedgedoc.org/. - Source: Hacker News / 8 months ago

Maybe Hedgedoc will fit these needs? You can use markdown to format. https://hedgedoc.org/. Source: 12 months ago

If self-hosting is an option for you I would recommend that you go with HedgeDoc. Completely open source, you get all the features you asked for including real time collaboration. Source: 12 months ago

You can give HedgeDoc (https://hedgedoc.org/) a try as a replacement for Google Docs. It is the one that works best for concurrent editing IMO (but it is markdown which can be a problem for some). - Source: Hacker News / about 1 year ago

I use one singular HedgeDoc document for that purpose. It's not exactly the same intent as Google Keep, but it's an awesome project I use anyway and fills the role perfectly for me personally. Source: about 1 year ago