As activity on this thread is stagnant at present, I thought we might pursue the 'book' biz a little further.
'Megillah' as Hebrew for scroll developed shortly before the Babylonian exile-'Ancient Scribes and Scripts and the Clues They Leave'--Richard Hanson.
I haven`t had time to mull it through yet, but my thinking goes somewhat along these lines---
The longest scroll found to date measured 32 feet (67 columns?) which must limit the length of the 'book' contained within, therefore the 'book'/scroll found by Hilkiah must have only contained a limited amount of the Torah (usually believed to be Deuteronomy). Therefore, where were the other books of the Torah found?