I used to COPS on my nas. I would add books using calibre, stored on the nas, and served up via COPS. Many apps (I used FBReader) have OPDS capabilities. This is a sort of online catalog which your reader can browse, search, and download books from. I almost never used the online reading feature, but did utilize the catalog concept quite a bit, especially when switching devices. I had a rooted Nook that had both FBReader and Kindle reader installed, giving me a sort of "best of three worlds" reader. I did have to "page sync" manually. I would do this by memorizing a sentence or phrase on one device and then search for it on the other device.

https://github.com/seblucas/cops https://wiki.mobileread.com/wiki/OPDS