Books written up in Latin so not easy to read (5)
I believe the answer is:
stoic
'not easy to read' is the definition.
I can't judge whether this defines the answer.
'books written up in latin so' is the wordplay.
'books' becomes 'OT' (Old Testament).
'written up' says the letters should be written in reverse (in a down clues, letters go up).
'in' indicates putting letters inside.
'latin so' becomes 'sic' ('so' in Latin. I am not sure about the 'latin' bit.).
'ot' written backwards gives 'to'.
'to' inserted within 'sic' is 'STOIC'.
Can you help me to learn more?
(Other definitions for stoic that I've seen before include "Phlegmatic type" , "he's not complaining" , "One patient under misfortune" , "One who endures hardship uncomplainingly" , "One able to endure hardship; philosopher" .)