In the past few days, has started showing that a new edition of Letters from Father Christmas is planned for release on September 19, 2019 - one year from now. The details are of course quite sparse and contradictory (the listing says it is both a paperback and a deluxe slipcased edition) so I asked HarperCollins if there was any news they could share. The response I received was
It’s true - FCL is joining our range of uniform deluxe editions.

We will have to wait for more information, but hopefully the art reproductions will get the deluxe treatment and we can enjoy adding this to our collections!

Info (subject to correction of course):
Letters from Father Christmas
Deluxe Slipcased edition
192 pages
Planned release date: 19 Sept. 2019
ISBN-10: 0008327726
ISBN-13: 978-0008327729

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Letters from Father Christmas: Deluxe (UK, HB)

HarperCollins (2019-10-17)

£75.00 HarperCollins (Hardback) - Availability: Click to check
$63.95 Blackwell's (hardback) - Availability: Usually dispatched within 7 days
$71.22 Biblio (Hardback) - Availability: Click to check
$66.49 (Hardcover) - Availability: Now
£35.52 (Hardcover) - Availability: Now
€63.66 (Gebundene Ausgabe) - Availability: Now
£66.33 AbeBooks - Availability: Click to check

(Thanks to Jeff Hawboldt for pointing me to the Amazon listing!)