Stellar Library consists of three components which are connected using the public internet. These components are the Library Manager that runs in a web browser on your computer, the reader App that runs on the mobile device and Stellar Library's Servers which are hosted by Amazon Web Services. All communications between these components are encrypted using industry-standard SSL/TLS using strong ciphers such as 2048bit RSA and 256bit AES.
Amazon Web Services (AWS) has a strong commitment to security and follow industry best practice. For more details about security at AWS see http://aws.amazon.com/security/.
For some examples of other companies that trust AWS see: http://aws.amazon.com/security/.