Dear Davide,
As a monograph on Syriac Christianity in China, I would still recommend the one by Nicolini-Zani (link below), especially to an Italian reader. An updated English translation ("The Luminous Way to the East") came out in 2022, which I unfortunately do not have as PDF.
Though beginning to be a little out of date, the biography in the attached article of mine (see esp. note 1) will hopefully still be useful.
One important item not mentioned in that biography (for obvious reasons) is: Alexis Balmont, Le christianisme chinois du haut Moyen Âge : Recherche historique, philologique et théologique sur les textes chrétiens chinois du VIIe au Xe siècles, Paris, Éditions du Cerf, 2025.
Best wishes,
Hidemi