After 39 years, I finally found the path to the solution (book version).

The overall solution is based on using the map to assemble what amounts to a quasi-mechanical device, combining the results of extracting relevant strings of letters from the body of the text in several chapters, after manipulating this text by following rules hidden in the double meaning of sentences. A large number of sentences are self-referential. While telling a coherent story these sentences provide raw text to be manipulated and information about how it should be manipulated.

Since the solution involves the manipulation of sentences/words or even letters, you have no way of knowing where it will lead until the end, where the letters left add up in specific ways (after conversion to numbers) to give latitude and longitude in degrees, minutes, seconds. This is likely one of the most sophisticated puzzle ever made.

This short video is only a teaser. It points toward the correct path without providing solutions to the several puzzles that must be solved.

The following document (pdf) provides the full, step by step solution: