The Revenge of the Invisible Giant

Years ago the Fjurge Brownies fled pursued by a stone giant bearing the seedling of the Wyrdie Tree in Dundoodle. When he hears that the brownies lost their magical BOOK OF EARTH in the process Archie McBudge is determined to find it for them. Only trouble is the book was locked in a tunnel - the key to which was broken into four pieces. Archie Fliss and Billy set out to find the pieces of the key but why was the tunnel sealed off in the first place? And what is the deep sinister MOUNTAINOUS voice Archie keeps hearing on the wind?