Brihat Parashara Hora Shastra (2 volumes)

Everything in Parashari astrology traces back to this text. Sage Parashara's treatise covers the signs, the houses, the divisional charts, the dashas and the yogas — and every later author is, in some sense, commenting on it.

This is the two-volume edition with the Sanskrit verse, transliteration and English translation set side by side, so you can check a translator's choice against the original.

Not a beginner's book. Read a primer first, then come here and stay for years.