From a tale I'm reading:
The chances of experiencing it are so small as to be beyond the notice of any native guidebook =
Le possibilità di sperimentarlo sono tanto piccole quanto ...(?)... di qualunque guida locale.
To paraphrase: the chances of experiencing it are so small that it is not even noted/mentioned in the native guidebooks. In other words, it does not meet the standards of the guidebooks for being included in them. It is beyond the limits of what is considered noteworthy.