Some Useful References

Free Haven Project's Papers
https://www.freehaven.net/papers.html
Or on the scary "darknet"
http://7fa6xlti5joarlmkuhjaifa47ukgcwz6tfndgax45ocyn4rixm632jid.onion/papers.html

Tor Project
https://www.torproject.org/
http://expyuzz4wqqyqhjn.onion/index.html
(v2 onion services, as opposed to v3 above)

Bad VPNs
https://blogs.harvard.edu/hroberts/2009/01/09/popular-chinese-filtering-circumvention-tools-dynaweb-freegate-gpass-and-firephoenix-sell-user-data/