1. A server system comprising: one or more processors; and one or more computer-readable storage media storing computer-executable instructions that, when executed on the one or more processors, cause the one or more processors to perform acts comprising: storing, by the server system, annotation data from a first electronic book (eBook) reader device to a data store; determining that the annotation data is specific to a digital work; determining that an invariant location reference identifier is assigned to the annotation data, wherein the digital work is partitioned into a plurality of segments, a first segment has the invariant location reference identifier assigned thereto, such that the invariant location reference identifier is uniquely assigned with the first segment of the digital work, regardless of display conditions used to display the digital work; receiving a request from a second eBook reader device for the annotation data; synchronizing the annotation data stored in the data store with annotations stored on the second eBook reader device; determining that the second eBook reader device has presented a valid authorization credential for receiving the annotation data; and sending the second eBook reader device the annotation data.
4. The server system of claim 1 , the acts further comprising synchronizing content stored on the first eBook reader device with the data store.