1. N. Bourbaki, Groupes et algebres des Lie, Elements de Mathematique, Paris, 1981. (Chapitres 4,5 et 6).
2. Central extension of groups of Lie-type;Curtis;J. Reine Angew. Math.,1965
3. D. Gorenstein, R. Lyons, R. Solomon, The Classification of Finite Simple Groups, No. 3, Part I. Almost simple K-groups; Mathematical Surveys, and Monographs, 40.3, AMS, Providence, RI, 1998, (Chapter A).
4. B. Mühlherr, On the simple connectedness of a chamber system associated to a twin building, Preprint.
5. F.G. Timmesfeld, Abstract root subgroups and simple groups of Lie-type, Monographs in Mathematics, Vol. 95, Birkhäuser, Basel, 2001.