You can start with the chapter in the second volume of Kobayashi-Nomizu; both volumes are written in a reader friendly manner, and normally proofs are well detailed with precise references to the previous used results. Helgason is quite tough I think. Neither of these two are geometric enough in my opinion. The best for that are Loos's books (Symmetric space 1 and 2); these are my favourite !
Amin
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