The Picard novels are really good (at least the first two, I haven't read the other ones yet!). The Last Best Hope is the general backstory of the show up to Picard quitting and does a really good job of weaving all off the different threads together. Personally, I enjoyed it more than the show. The Dark Veil is an adventure on the Titan after the synth ban came into effect and is a bit more stand-alone, while also implying some interesting things that you'll probably notice if you've already seen the show.
For DS9, I can recommend "A Stitch in Time", which is basically a non-linear Garak autobiography written by Garak actor Andrew Robinson, in the form of letters to Bashir.
Then there's "The Never-Ending Sacrifice", which is about the Cardassian child that got adopted by Bajoran parents and then turned out to be the lost child of some politician in that one DS9 episode. It basically chronicles what happens on Cardassia throughout DS9 and in the first two years after the war through the eyes of an outsider and it's absolutely fantastic. If you want to read just one Trek book, I'd say read this one; it paints a very detailed and interesting picture of Cardassian society, its regressive and revolutionary elements, its politics and culture. It's just great.
And the "obligatory" recommendation for DS9 is of course the DS9 relaunch, which is basically a bunch of books set in the year after "What You Leave Behind", continuing the story of the remaining characters (no Worf or O'Brien), as well as introducing some really interesting new ones (like Shar, Prynn and Vaughn). The reading order for the first batch of books is as follows: Avatar Book One and Two, The Abyss, Demons of Air and Darkness, the Mission Gamma mini-series, Rising Son, Unity, the Worlds of DS9 mini-series, warpath, Fearful Symmetry and The Soul Key. After that it gets a bit messy (and the quality takes a bit of a dip), but imo it's still worth it to read these books.