The Gateways series had a weak framing story, but don't let that put you off "Demons of Air and Darkness". More importantly, its the fourth book in the acclaimed series of DS9 novels published since 2001. DeCandido accurately depicts Kira, Bashir, Ezri, Quark, Nog, Kasidy etc and focuses on Taran'atar, the Jem'Hadar assigned to Deep Space 9 by Odo in "Avatar".
The story was followed up in "What You Lay Beyond", in ths short story "Horn and Ivory". If you have trouble finding a copy, both "Demons of Air and Darkness" and "Horn and Ivory" are available in the Twist of Faith omnibus.