I have heard that I can use calibre to create a cloud with all my books, not just ones from a specific vendor. How would I do it?
Yes, it is possible and rather easy to do so.
You start it with the
connect/share button on the main toolbar, then selecting
start content server; it can be configured from
preferences > sharing. It should be a simple task, if you need more help just ask. With this method calibre must be running because basically what you are doing is setting up a web interface to connect to it. This way it will be accessible from any web browser. See also calibre manual on this topic.
I also suggest you to read this guide from mobileread.
Be sure to protect your content server with a password, so you must be required to login to be able to access it. You don't want to let everyone access your copyrighted ebooks and have to do with piracy.
Alternatively, if you have a server available, with PHP, you can install COPS (https://github.com/seblucas/cops/releases) on the server, and use
rsync to regularly copy your entire Calibre directory to the server. Then you won't need to have Calibre running full time.
Again, it makes sense to protect it with a password.
You can definitely create your own eBook library by combining Calibre with other solutions to ensure your books, highlights and comments are reliably accessible online which would require a lot of overhead.
I had a similar problem when we set out to build BookFusion. BookFusion allows you to easily organize, store and read your eBooks and other documents across all devices. You will be able to access them in the cloud while accessing them on any device(Android, IOS and Web). In a gist your own personal ebook cloud library. To learn more visit https://www.bookfusion.com/reading/cloud-library
We also have a Calibre plugin to make creating your cloud library easier. https://www.bookfusion.com/reading/calibre
PS: Founder at BookFusion. Please feel free to respond or DM any feedback or suggestions.