If you haven’t read them…you may wish to read:
The Bhagavad Gita
The Ashtavakra Gita
(both The Essence of the Ashtavakra Gita, and The Essence of the Bhagavad Gita, by Ramesh S. Balsekar, are excellent)
You may also wish to check out the teachings of these sages:
Ramana Maharshi (any works or talks by this sage of Arunachala, Tamil Nadu; see also Pointers from Ramana Maharshi)
Nisargadatta Maharaj (Bombay; I Am That: Talks with Sri Nisargadatta Maharaj; Consciousness and the Absolute, et al)
Ramakrishna Paramahansa (The Great Swan) from Dakineshwar, Calcutta; Ramakrishna was also a bhakti sage, devoted to the Divine Mother Kali.