I've been using an ebook reader for a while and was looking for a lightweight solution to share epubs from my desktop computer to my handheld device easily.
Calibre is well-known and provides much more features, most of which I don't need, hence the hunt for something lighter.
This sparked the creation of bookshelf, a free OPDS catalog server. It also exposes a webview and convenient search interface.
The code is open source.
Bookshelf is fairly minimalist: it will index a directory and serves your files through either OPDS catalog or web interface.
The way I deployed it, it indexes a directory shared through SFTP where I can conveniently upload epubs. A systemd timer triggers a file index every now and then.