These are my personal Greats:
DS9:
Terok Nor: Day of the Vipers by James Swallow. The first of a trilogy the book is stand alone. The young Gul Dukat machinates his rise to power as the occupation of Bajor begins with insidious overtures of goods and friendship to the corruptable Bajor ruling class be they priest or politician. We see this through their eyes and through the eyes of characters who are regular Bajoran people. My favorite Trek book.
A Stitch in Time by Andrew Robinson. Garak's biography, written by the actor that played him. Considered one of Treklit's greats for good reason. Gripping and believable portrayal of Cardassian society with its almost medieval social mores. Difficult to find now, if you ever see it grab it!
VOY:
Mosaic by Jeri Taylor. Janeway's biography! A great and interesting read, a must read for any Janeway fan. Considered canon in some parts by some people, events in the book are referenced in the show itself as it is written by the creative brain and co producer of VOY.
Pathways by Jeri Taylor. This is everyone else's biographies, told via a contrived "we are trapped in an alien death camp let's all share" framework. Even the characters I didn't care for when I read this had great stories and background. Very interesting and enjoyable read.
Homecoming and The Farthest Shore by Christie Golden. These are the first two books of VOY relaunch, Homecoming begins like five minutes after Endgame concludes. Quick reads and light they have authentic voices of the characters (if you've read Treklit where this is not the case this is a big deal

). They are fun and you can read them without continuing on into the multi book sagas that follow. They deal with the crew's adjustment but also immediately jump into a new story. The books that follow these two I thought were woeful and having read Golden in another fandom (Warcraft) it seems she comes out of the gate running and then just gets sloppier and shoddier writing and ideas with each subsequent novel. I've reread these first two a couple times.
I have other ones I liked (the next two in the Terok Nor trilogy are fine books) but these would all be great to start with in Treklit. If you are interested in the Eugenics wars the story of Khan as told by Greg Cox has several books to it which are great too, quite long.