Piaget, Jean: The Language and Thought of the Child https://www.amazon.co.uk/Language-Thought-Child-Routledge-Classics/dp/0415267501/ (Routledge Classics, 2001)
Piaget, Jean: Psychology Of The Child https://www.amazon.co.uk/Psychology-Child-Jean-Piaget-x/dp/0465095003/ (Basic Books, 1969, 1971, 2000)
We can further recommend this site: http://www.piaget.org/
Except for Kegan’s descriptions of Kohlberg’s model, the sources we have used regarding Kohlberg are all online presentations of Kohlberg’s model, not texts by Kohlberg himself.
Kegan, Robert: In Over Our Heads: Mental Demands of Modern Life https://www.amazon.co.uk/d/Books/Over-Our-Heads-Mental-Demands/0674445880/ (Harvard University Press, 1994)
Kegan, Robert: Immunity to Change: How to Overcome It and Unlock the Potential in Yourself and Your Organization (Leadership for the Common Good) https://www.amazon.co.uk/Immunity-Change-Potential-Organization-Leadership/dp/1422117367/ (Harvard Business Schoo Publishing Corporation, 2009)
This Wikipedia article gives a proper representation of Kegan’s model: https://en.wikipedia.org/wiki/Robert_Kegan
Circles of Solidarity
Welzel, Christian: Freedom Rising: Human Empowerment And The Quest For Emancipation, 2014.
Gärdenfors, Peter: Den meningssökande människan (The meaning seeking human), Natur och Kultur, 2006
Haidt, Jonathan: The Righteous Mind, Vintage Books, 2012