OAuth guides
http://stackoverflow.com/questions/3514579/can-anybody-explain-oauth-to-an-idiot – the second post has a good explanation
http://www.thebuzzmedia.com/designing-a-secure-rest-api-without-oauth-authentication/ – uses HMAC. Good read about security and authentication.
http://www.cubrid.org/blog/dev-platform/dancing-with-oauth-understanding-how-authorization-works/ – understanding how authorization works
http://marktrapp.com/blog/2009/09/17/oauth-dummies/ – dummies guide to OAuth
SSO
http://www.sitepoint.com/single-sign-on-explained/ – SSO explained
http://merbist.com/2012/04/04/building-and-implementing-a-single-sign-on-solution/ – Building and implementing a single sign on solution