A great authentic ancient book that just keeps on speaking to me through the years is the Tao Te Ching. Read sections of it daily and ponder what it might mean. My understanding of what this book is trying to convey has deepened with time. I like the old translations by Gia-Fu Feng and Jane English that are more than 25 years old as they feel more authentic. But I would get various translations and see what speaks to you.
Other books for your consideration:
The Song of Ribhu I Am That by Sri Nisargadatta Maharaj Ashtavakra Gita - (I enjoy "The Heart of Awareness" by Thomas Byron) Excelscia, The Home of the Gods by Arion - totally out of print and hard to find, but a great book.
Those are my first picks for inspiration.
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