To locate the PDF, consider the following steps:

If your North Node’s ruler is retrograde, write an “unsent letter” to a past-life version of yourself who failed to use that planet well. Then write a “letter from your future self” who has mastered it.

In a past life:

In this life:

The second half of the volume shifts from retrogrades to the broader framework of reincarnation. Here, the author (often attributed to the lineage of Martin Schulman or similar 20th-century karmic astrologers) presents a unique model: The Zodiac as 12 Stages of Soul Learning.

The PDF contains a step-by-step formula for releasing past-life karma using retrogrades:

For example, a Mercury retrograde in the 3rd house (communication with siblings/neighbors) might involve writing a letter you never send, then choosing silence instead of compulsive explanation.