I use Calibre-Web (self-hosted web-UI for Calibre) with my Kobo Forma, and with a quick change of a config file on the device (by plugging it into PC) I can use Calibre-Web as an online book store and retrieve my books over my network, I'd recommend it for anyone interested in keeping a large library and wanting to manage what's on-device.

No jailbreak or anything required, just some config in Calibre-Web and replace a URL or two in a config file on the Kobo.