My pastor mentioned that, as it concerns the broad history of Church liturgy and feasts and observances, he has done a lot of research and has amassed a understanding from various places, but doesn't know of much material that focuses just on Christmastide.
Two books he mentioned in particular that may be of interest to you:
https://www.amazon.com/Origins-Feasts-Seasons-Christianity-Collections/dp/0814662447?tag=ustxtadsp-20
https://athanasiuspress.org/products/for-signs-and-seasons-a-primer-on-the-church-calendar
First book is very academic and a bit dry but he spoke highly of it. If I understood him correctly it gets into how the dates were chosen for various holidays like Christmas and Epiphany and how, over time, things became more standardized.
He said he has not read the Signs and Seasons book yet but likes the author.
He also mentioned that because of some Protestant history rejecting the feast days and certain holiday observances like Christmas it is harder to come by Protestant material on the larger subject and that more is to be found among the RCs and EOs.
Hope this is a helpful direction for you. As I dig more for my own edification I'll let you know if I find anything else that is notable.
