Swamiji has written many insightful books, and articles.Articles
a) Study the 10 major Upanisad with Shankara bhasya
order suggested: Katha, Mundaka, Isa, Taitriya, Chandhogya, kena and later Mandukya, Brahardarankya (Aitreya is optional and needs to be taught by guru).
a) Read Vivekananda's books
b) Rama Tirtha