You're better off asking this in the TrekLit room, which is specifically dedicated to questions like this. But in the meantime:
There are two 'phases' of DS9 books - ones set during the series, and ones set after. The so-called
DS9 Relaunch book series has been massively massively popular, for the simple fact that they are allowed to make the kinds of big status quo changes that the series regularly did, without having to worry about being contradicted by future TV episodes.
The 34th Rule and
A Stitch in Time are definitely recommended for stories set during the series (although ASiT is actually framed just after the series, with flashbacks to many time periods). But other popular choices are
Hollow Men and
The Never-Ending Sacrifice, both by Una McCormack, and both highly regarded. Una is generally to go-to gal for Cardassia-based stories, and these are both excellent.
Moving onto the post-series novels, a good place to start is
Avatar pts 1 and 2, which is the literal 'relaunch,' the first story in what people have termed 'season 8', and the book that basically started the modern era of Star Trek novels. Further along the line, highlights include
Twilight and
Warpath, although you have to bear in mind that many of these stories are quite serialised, and would be best enjoyed as part of the whole series than as standalones. And even further ahead, there is the
Plagues of Night /
Raise the Dawn duology, which is pretty damn eventful, but I don't advise jumping ahead that far without reading some of the earlier stories first. As I said, very serialised.
If you go to the TrekLit room, you will find several other threads on this and similar subjects, and some posters have created massive flowcharts to explain the various book series, how they intersect, and what order to read things in. It might seem like a huge undertaking and rather intimidating, but the posters will happily guide you. We all enjoy welcoming noobs to the world of TrekLit!
.