I think this counts as a spin-off, in the vein of e.g. Mrs Bradshaw's Handbook or The World of Poo, or the Compleat Discworld Atlas - their contents weren't written by Terry (although he did approve the ones that were published during his lifetime)
I recall that Terry specifically gave Rhianna the right to write more Discworld.
She said she wouldn't, but this is just a spin-off.
This cover would be fine. The version just revealed is not.