Thanks to Zebracak3s for sharing in the r/warcraftlore discussion thread.
In the past Blizzard has delayed novels to line them up with important in-game events if the books contain lore reveals, for example, World of Warcraft: Exploring Azeroth – Eastern Kingdoms and Before the Storm.
The Sylvanas novel could be delayed to line up with a future patch, perhaps 9.2, or the BlizzConline 2022 event, as this novel likely contains spoilers for future patches, or even expansions. The Final Fantasy 14 expansion, Endwalker, launches on November 23rd, so Blizzard may have initially planned a fall release for Patch 9.2. With the Shadowlands release schedule seeming to have fallen behind at this point, possibly due at least in part to Blizzard’s legal woes and the effort that the developers are putting in to clean up the game, the Patch has likely been delayed from its original projected release, necessitating a publication delay for the book.
This novel is penned by Christie Golden, a veteran World of Warcraft novelist who has already spent time in Sylvanas’ head in other novels including War Crimes, the Mists of Pandaria denouement, and Before the Storm, the lead-up to Battle for Azeroth. Novelist Golden is also employed by Blizzard on their narrative team, and is therefore in a superbly unique position to to write the out-of-game stories of these beloved Warcraft characters. The audio version of the book is read by Patty Mattson, the actor who provides the voice of Sylvanas in the game.
As of this writing there has been no official statement from Blizzard on this topic.