Higher-dimensional algebra. VI: Lie 2-algebras. Baez, John C.; Crans, Alissa S. — 2004 Theory and Applications of Categories [electronic only]
Higher-dimensional algebra. V: 2-Groups. Baez, John C.; Lauda, Aaron D. — 2004 Theory and Applications of Categories [electronic only]
Higher dimensional algebra. VII: Groupoidification. Baez, John C.; Hoffnung, Alexander E.; Walker, Christopher D. — 2010 Theory and Applications of Categories [electronic only]