Hindsight is when we look back on an event with the knowledge that we have now. So the book wasn't written at the time of the author's journey, but some years later. There is an inference that the time lapse may have had an effect on what he eventually wrote.

Hindsight means understanding something after it happened, so "a work of hindsight" here means Lost City of the Incas is a book written after the journey that it's about and includes an understanding of that journey that is a result of time and reflection of many years.