Open-source ebook library management application for Windows, macOS, and Linux.
Calibre is a free and open source ebook library management application. It runs on Windows, macOS and Linux. It supports many formats and can convert ebooks between them. It can synchronize to many devices.
Calibre has plug-in interfaces for several features (conversion, downloading to devices, news feeds), which can be programmed in Python.
There is also a support forum on mobileread.com.