Sorry DDB, but I have looked around some more and haven't found anything with a sooner date than what I said. It looks like they are in fact doing this to us. :( Thanks for getting a better quality image. On second thought, I think that the cover does have a few relevant details. If you look at the phraxcraft, it has a little jet of phraxfire coming out of it, which seems to confirm the idea that the phraxchamber supplies propulsion while the wood gives lift. Also, the weird creature in the top right looks similar to an Edge wraith but it isn't, which seems to confirm that there are other lifeforms down below the Edge. If Celestia is on the official cover in descent equipment, then I think that we can be sure that she is going on the expedition (no guarantees that she lives though)
Everywhere (Amazon, Penguin Book etc) I looked it has the 9th of March, 2019. That's still over a year away! Maybe there were some issues we don't know about. The original publishing date was supposed to even be in 2017, wasn't it? The Edge Chronicles Facebook (as always) hasn't done the cover reveal yet, but they were also late the last time. Anyway, having a complete cover is a good thing. And I am very sure Chris a Riddell will post some more images over the course of the next year and then we'll have more to speculate about.
Having read WNW, I now have a couple more things which I think I can add to this conversation. I think that Forden and Gart's journey to to the Eastern Woods in WNW will be very similar to the journey to The Edge in The Descenders. We know the Midwood Decks (and therefore the who Midwood area) will be making a significant appearance in TD, so it's likely that that's where the tallow hats will appear, just like in WNW. I now know that the tallow hats were the slavers who lived in the floating fortress, so that's probably where they will appear as well. Since they'll have to pass over the Twilight Woods to reach The Edge, it's possible that they might buy some illicit stormphrax from a mine, just like in WNW. I think it's unlikely that Celestia will die, because Alcestia dies in the "Doombringer" part of WNW.
Here's a transcript of the visible text in the image:
Chapter 16 ...the phraxlighter...
"Quiet, please," she said, her voice low and urgent. "Twilight is upon us and Tug must work quickly." With the sun low and the light fading, the stacks of buoyant rocks, each boulder larger than the one below it, were casting long shadows across the stone pavement. ...[visi]tors to the Stone Gardens, whi[te ravens]... ...[their?] wings outstretched... ...ra[ucou]s...
The italic text almost certainly means that the "speaker" is a waif, communicating using telepathy. This waif could be Sentafruice, who has appeared in several other images. The fact that twilight is significant could mean that they are moving/using stormphrax, and need the twilight to be able to work safely. Tug, the waif and the main character (probably Cade) likely used a phraxlighter to reach the stone gardens. The rest of the text is the usual description of the Stone Gardens and the white ravens, and isn't very important.
There are some notes visible in the top right hand corner; here's a transcript of those: