Sigurdur Helgason, Group representations and symmetric spaces, Proc. Internat. Congress Math. Nice 1970, Vol. 2 book no 10, Gauthier-Villars (1971) 313-320 [pdf, pdf, djvu]

Sigurdur Helgason, Differential geometry, Lie groups and symmetric spaces, Graduate Studies in Mathematics 34 (2001) [ams:gsm-34]