Re: DS9 - Where to start?
As for book seven, well that's different. It's a short story collection that brings together the codas from the six main books. The DS9 portion of that is fairly necessary, but I suppose if you really really had to, you could skip it, at least plot-wise, especially if you don't give a toss about the other five. Character-wise though, it's very much worth it.
How it worked out was the novels all ended on a kinda cliffhanger that was then continued on in the last book's stories. Most of them are fairly separate stories, but they all pick up from the cliffhangers and build off of the novels.
Over the course of many encounters and many years, I have successfully developed a standard operating procedure for dealing with big, nasty monsters. Run away. Me and Monty Python.
Harry Dresden - Blood Rites (The Dresden Files #6)