The Complete Works of Rudolf Steiner
The Foundational Works:
Rudolf Steiner's work contains many interesting facets as well as some truly esoteric persectives. But his main effort was always to be understood. To that end he wrote several introductory books to explain how he sourced and developed his thought, books which he revised continuously throughout his life. These Foundational Works are introductory in the sense that they presuppose no prior knowledge or familiarity with his other works. They are not easy reading, nor were they meant to be. Steiner was sourced in a long tradition of idealistic philosophy, and his models were Hegel and Schelling. But they were written to be comprehensible with the requesite effort and are very clear expositions of the basics of his worldview.
And there are many more windows into the Complete Works on the main Complete Works page.