Several chapters from the Encyclopedia of Human-Computer Interaction are freely available online under open access.
Dix, Alan J. (2010): Formal Methods. In: Soegaard, Mads and Dam, Rikke Friis (eds.). “Encyclopedia of Human-Computer Interaction”. Aarhus, Denmark: The Interaction Design Foundation. Available online athttp://www.interaction-design.org/encyclopedia/formal_methods.html
Interaction-Design.org “Interaction-Design.org is a new type of encyclopedia! It’s free, it includes HD videos, commentaries, and lots more. All chapters are written by leading figures within each subject.”