Calibre is usually the go-to open source ebook reader. I don't know about SaaS but it does include a server so if you keep you ebooks centrally located and acess them from there, your bookmarks and notes would be preserved.
Sorry for OT, but there are no PMs on hacker news. I found your comment about CO2 sensors for Raspberry PI from last year [0]. Have you finished this project? If yes, could you share a few details?
For my first Rasberry PI project, I am considering buying Enviro for Raspberry Pi + Air Quality. I would like to add an affordable CO2 sensor that would require minimal config and no soldering. Pimoroni folks told me MH-Z19 would not work because of the number of pins. Would you have any suggestions on what sensor to choose and how to make it work with Enviro / Raspberry PI?
https://calibre-ebook.com/about