I think the OP is referring to the ad for a (then) upcoming novel on the last page of the Twist of Faith omnibus - it's an early promo cover for
Fearful Symmetry, which looked like this:
http://memory-beta.wikia.com/wiki/File:FearfulSymmetry2.jpg
The book eventually changed its author - from Leanna Morrow to Olivia Woods - and was eventually not published under the "Mirror Universe" logo.
http://memory-alpha.org/en/wiki/File:Fearful_Symmetry_cover.jpg
Fearful Symmetry is one of the last DS9 relaunch novels published so far, so it's definitely not what you should read next. The latest novels in the DS9-R contain a Mirror Universe-related plot.
DS9-R books consists of the following books:
books that are not part of the main plot:
The Lives of Dax (published a few years before the rest of the DS9-R, the framing story is set shortly after
What You Leave Behind, it doesn't affect the rest of the relaunch so it doesn't matter when you read it, but it's a good book)
A Stitch In Time (a great novel by Andrew Robinson about Garak, it doesn't matter much when you read it relative to the other books, but it's probably better to read it before the Cardassian story in
Worlds of DS9 and before
The Never-Ending Sacrifice)
The Left Hand of Destiny, parts 1 & 2 (a two-part novel about Martok and the Klingon Empire, although it was published after
Rising Son, chronologically it takes place right after What You Leave Behind, but it doesn't affect the rest of the relaunch)
The Never-Ending Sacrifice (a side story about Cardassia - also a great novel) This one was published last year so you can easily find it, the others are out of print.
books that are a part of the main DS9-R plot:
[those you've already read in Twist of Faith:
Avatar parts 1/2
Abyss
Demons of Air and Darkness
Horn and Ivory]
Mission Gamma 1: Twilight
Mission Gamma 2: This Gray Spirit
[both are reprinted in These Haunted Seas omnibus book, which you can still find it print.
Unfortunately, the next books are out of print:]
Mission Gamma 3: Cathedral
Mission Gamma 4: Lesser Evil
Rising Son
Unity
[The next books you can probably still find:]
Worlds of Deep Space 9, volume 1: Andor and Cardassia
Worlds of Deep Space 9, volume 2: Trill and Bajor
Worlds of Deep Space 9, volume 3: Dominion and Ferenginar
Warpath
Fearful Symmetry
The Soul Key
You may also want to read the anthology
Prophecy and Change - all the stories but one are set during the show, but the last story, Andrew Robinson's "The Calling", is his sequel to
A Stitch in Time. It may be quite confusing, though (it certainly was for me), since there are lots of references to the events of a play called
The Dream Box that Robinson wrote and performed with Alexander Siddig at conventions, but it was never published anywhere. (The mini continuity would be: A Stitch in Time - The Dream Box - The Calling.) The story is set a few years after the rest of the relaunch and contains a few big revelations about what happened on Cardassia.
There are also two comics which are a part of this continuity, but there are not really important for the books. though you may be interested to see what the new characters look like in the comics
:
K-Vector, set after the DS9 finale and before
Avatar
Divided We Fall, set after
Abyss
