My history of the Constantinian Order is now available in English, at a pre-publication price available until 28 December (which includes packing and postage to the US or Europe - I am afraid it is pretty steep as it is 580 pages with 300 images, on a large format and heavy). It has been published by the Boletin Oficial del Estado, and can be bought online at the link below. There is a link to a PDF on the BOE page at the link below of some pages which includes the cover, contents pages, and some extracts from the book.
see:
https://tienda.boe.es/detail.html?id=9788434025066 There is also a Spanish language edition, but I have not yet got the link for that.