We've discussed this before somewhere. The numbers are "military" or "universal" grid references. The six digit number is an abbreviation of the actual UTM coordinates - the first three numbers are from the UTM easting and the second three numbers are from the UTM northing. You will need the original map from which the grid was derived to locate position. Do they mention a map in the text of the book ? i.e a 25K or 50K topo map most likely.
To find yourself, think for yourself © Socrates 469 BC