Quattro pezzi sacri is a collection of four small religious works, published at the end of the composer’s life, but written earlier. The oldest, the Ave Maria is a bit more special than the rest, and should not be performed with the other three, said Verdi. That might have happened only the first time, because it is always done as a set of four.
That Ave Maria is written in a so called enigmatic scale: C – D-flat – E – F-sharp – G-sharp – A-sharp – B – C. Enigmatic scales use elements from both minor and major scales, so the result sounds a bit out of place here and there. That makes listening to it an unexpected delight for me. It is almost experimental, which I suppose in a way it is.
Verdi stopped writing operas for ten years after Aïda in 1874. He was in that time the most celebrated opera composer alive, and his productions were running everywhere. However popular that made him, at the end of his life he returned to where he once started: church music.
A curious tidbit of information on wikipedia: Verdi wanted to be buried with the score of the Te Deum. So. Was he?