I recently read all the Deep Space Nine Relaunch books (a series set after the show ended) as they vary somewhat in quality. But the best one was A Stitch In Time, which is kindof a biography of Garak, written as a series of journal entries included in a letter to Dr. Bashir. What’s most impressive is the whole thing was written by Andrew Robinson, the guy who played Garak, and not a professional writer.
Other good ones are The Lives of Dax, Avatar (two books), the Mission Gamma series, and the Worlds of Deep Space Nine series (except the boring shit about the Andorians. They introduced this Andorian character on DS9 and he’s an uninteresting whiny git and takes up way too much time in the series.) The Warpath, Fearful Symmetry and Soul Key arc is also pretty good.
Rising Son was terrible; probably because it is about DS9’s least interesting character (Jake) and an annoying sentient dog. I also found the Millennium trilogy to be execrable.
So after I finished all those, I started reading the numbered series in order. (#1 is just a novelization of Emissary, so I skipped that.) #2 was pretty awful, but I found it interesting to see how the author treated the characters since by the time it was written, DS9 had only been on the air a few weeks. It’s amazing how many of these things they manage to crank out.
#4, The Big Game, is my favorite so far, and it involves an amusing side plot in which Quark holds a poker tournament.
#5, Fallen Heroes, is surprisingly dark. I mean, even for DS9. This book has more sadistic violence and brutal death in it than I’ve ever seen in any Star Trek media. Of course you know from the first page that it’s all going to hit the reset switch because time travel is involved.
I’m currently in the middle of #7, Warchild, which has some interesting stuff involving Bajoran prophecies. IIRC this one takes place in between seasons 1 and 2.