https://etherpad.org/ lets you set up your own instance or sign up to free existing instances for online document collaboration. It seems very on par with Google Docs.

https://www.collaboraonline.com/collabora-online/ has wide document format compatibility and is most comparable to M$ 360, but you need to sign up for a free demo and I don't know how long that lasts or what it costs afterwards.

https://cryptpad.fr/ This is the one I've personally used the most without ever creating an account. It uses OnlyOffice, is e2ee, easy to create and share. Without an account, your document is deleted after 90 days. With a free account, you get 1GB of storage and the ability to share folders and collaborate with other users.

The safest option is always just work in the offline editor of your choice and send it back and forth securely with edits or comments. Keep each revision saved separately so you can go back if needed.

Good luck with your book!

cryptpad All your docs are stored encrypted, so no AI can be trained on.

I'm afraid AI is gonna use my work so I wanna move away from Google.

That is a completely valid fear, from what I understand of how Google operates.

I no longer trust Google with anything.

i see many comments here suggesting other paid proprietary tools and just want to add that you should please consider exclusively FOSS solutions. replacing google with another company saying: "trust me, bro", just does not cut it anymore. the internet is filled with stories of professionals losing work, because insert big tech company suspended their accounts, trains their AI without disclosure or just leaked your credentials and apologizes by makung a whoopsie post and suggesting you just change your password.

the more open, local and established your setup is, the less you are going to have those headaches and can focus on actually working, even if it means a little more hurdles with setup and less flashy features.

don't be a sheep

Proton for a “drop in” kind of replacement, although there are some format shenanigans iirc. Nextcloud should work fine, and you can have it hosted on their servers or host yourself

Obsidian/any competent editor + sync thing + backup.

I haven't had any issues with this setup. The syncing is almost instant.

https://european-alternatives.eu/

In general this is a great resource for finding alternatives, obviously with a European bias. But many are FOSS as well

Proton is probably the easiest—their free tier might even be enough for your use case.

Amazed that I didn't see any references to OpenDesk. Specifically built on open-source to escape foreign government and/or corpo malfeasance.

Can be self hosted with k8s. https://gitlab.opencode.de/...

Depending on the nature of your needs wrt version control, and how you want your editor to provide annotations, that will shape what solutions work for you. Those are pretty important considerations you will probably want to clarify, to get good advice. Also your comfort with self-hosting.

I've personally switched to silverbullet. It offers some really cool features, but I don't think it's going to provide the things you want in a way I suspect you'd like.
For example, annotations would be directly inline, not in a little sidebar. And there is no versioning to tell you who made what edit, when; you have to self host it, and provide your own backups at whatever schedule you want, separately from the app. And the really cool features probably aren't going to be useful for the manuscript itself, it's things like smart linking to sections of pages, and the ability to embed search queries of your docs directly in a page.

Murena Workspace makes use of Nextcloud and OnlyOffice for their e/os phones. But you don't need to use e/is to sign up for workspace.

https://murena.com/workspace/

Dropbox?

I'll def post about it once it's done! Rn only my inner circle of irl buddies and my editor are reading it.

I see many great suggestions here so I am only gonna add one thing to them: consider self hosting if you care about privacy and owning your own work. Something like nextcloud has all of the features of google drive/docs built in and much more and these days really isn't that hard to expensive to setup.

There's an app called "Notebooks" available in Moss

It's free, open-source and online, but must run as an app and is still in Beta.

Paid versions of m365 keeps you in control of your work product.