Yes, I also think there is a reading audience for the Christmas season, but I also suspect some publishers will be in a rush to have something in the bookshops by the late summer or autumn. I think the latter category is concerned with catching the reader interest before too much time has past and they may have an eminent author on royal topics with something to share ahead of the rest of the more general coverage of the coronation day. I suppose we will know one way or another soon enough, since book authors and their publicists often also drum up interest weeks in advance of an actual release date of the book.
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There are a lot out there that were published prior to the Coronation. Which were sort of leading up to the Coronation. I've not seen any actual books about the Coronation or the coverage of it. Personally I would wait at least 6 months, possibly a year, before these books start to appear. I think most authors and publishers will be aiming for the Christmas market.