This is a stand in for all the essential texts of Kashmir Shaivism including the Shiva Sutras, Spanda Karika, Vijnana Bhairava Tantra, and Sri Tantraloka.
Kashmir Shaivism is the richest and most beautiful philosophy of life I've found anywhere. It provides an approach to enlightened living that's fully compatible with self-actualization through career (and modern society by extension), and emphasizes the inherent significance of artistic expression.
I read these texts for their general philosophical value, so I look past all the theistic symbolism and focus on the underlying meanings.
For those interested, I'd start by understanding non-dualism and Advaita Vedanta. I only fully appreciated Kashmir Shaivism with this context, since it synthesizes prior philosophies like Advaita, Vishishtadvaita, and Tantra.