FM Andrey Terekhov's Two Knights Defense repertoire on Chessable uses this variation against 4.d3.
That repertoire deserves to be more widely known: it's one of the best on Chessable, the author updates it frequently, and it's free!
In the introductory text of 4.d3 h6 he notes:
In the beginning, this line has been mostly used as a surprise weapon, but in the past two years it developed its own body of theory. GM Alexey Kuzmin published a couple of articles summarizing the latest developments in this line in "ChessBase Magazine" (in 2018) and in Russian "64" (2019) .
So those are sources too.
And according to a ChessPub discussion, there are also two analyzed games in this line in Alexander Ipatov: Unconventional Approaches to Modern Chess Volume 1; Thinkers Publishing 2019, although I don't see it in that book's list of contents.