Brittle letters added to one or more texts (8)
I believe the answer is:
libretti
'more texts' is the definition.
'libretti' can be an answer for 'texts'. I am not certain of the 'more' bit.
'brittle letters added to one' is the wordplay.
'letters' indicates anagramming the letters.
'added to' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'brittle' with letters rearranged gives 'librett'.
'librett'+'i'='LIBRETTI'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for libretti that I've seen before include "Words" , "Opera texts" , "Books" , "score-books?" , "Da Ponte's texts" .)