Timeless non-fiction book (4)
I believe the answer is:
ruth
This could be a double definition.
'timeless non-fiction' is the first definition.
The definition suggests a singular noun which matches the answer.
'book' is the second definition.
(book of the Bible)
Can you help me to learn more?
(Other definitions for ruth that I've seen before include "condolence" , "Old word for pity, or Bible book" , "Damsel" , "Girl's name; pity (archaic)" , "Babe -, US baseball player" .)