This is the 6909th most frequent Catalan word.
unnic
"Unnic" in Catalan means "unique" in English.
Here, 'únic' is used as an adjective meaning 'only', modifying the noun 'llibre' (book).
Aquest és l'únic llibre que tinc.
This is the only book I have.
In this sentence, 'únic' functions as an adjective meaning 'unique', to describe the concert as special and one of its kind.
El concert va ser únic, espectacular!
The concert was unique, spectacular!
'Únic' here is used as an adjective to attribute 'uncommon and distinctive talent' to Joan, marking him as special.