In all the moving-to-America-and-getting-married thing, I totally forgot to tell anyone (including m'webmistress, I think) that House of Bells came out in the UK at the end of last month. I do have an excuse, mind, as I haven't actually seen it myself; I think they tried to deliver a boxful the week after I departed British shores, and it hasn't caught up with me yet.
Anyway: the book exists and may be purchased. It's not exactly a sequel to House of Doors, more another story in the same locale, in another time period (1960s, rather than WW2). But it does feature a scatter of characters from the first book, and thematically it is not utterly unlike.
This is it at Amazon; you may have your preferred place to shop.