Stored information: In order for the cloud sync function to work, this add on uses a randomly generated user id to store your data. This data includes user added meta information about the work (such as ratings and chapter bookmark), it also includes visit timestamps for each of the works you bookmark, as well as the url of the work itself.
Device information: As part of backing up the bookmark data, your device has to talk to the server. Some device information and IP address are part of that transaction (as they are for any website you visit).
Metrics: New Relic is used to understand in the aggregate how the servers are behaving; it is used to monitor this incoming traffic.