''I felt certain there must be gold in those hills, Jack'', wrote a prospector to Ion Idriess, ''but I know very little about the game.'' And so Jack Idriess wrote Prospecting for Gold in 1931. This is the 20th edition and known throughout Australia as the classic self-help manual for would-be prospectors. ''This book is written to help the new hand who ventures into the bush seeking gold... The "towny" prospector, with this book as a guide, will soon master methods of prospecting and the working of his find.'' In an easy conversational tone, the author of Lasseter''s Last Ride and Flynn of the Inland sets many a hopeful prospector on the road to discovering gold.