If you’ve not read any of the Dresden Files, I wouldn’t recommend you read this collection of short stories. I would recommend you go and read Storm Front, before you go and read all the others, /then/ read this. While the stories aren’t all directly tied to the main plot arc, there’d be a bunch of spoilers, and you’d be a little lost in what’s going on. They’re good books. Go read them. If you’re not completely up to date with the main novels, you want to get up to date before this, to avoid at least the massive spoiler in Aftermath (the last short story).
On the other hand, if you have read them, get this collection and read it. Many of the stories within have already been published in other collections (Aftermath is the only one that’s exclusive), but I know I didn’t have them all. The stories are all covering smaller incidents, generally between the main novels, and showcase Harry Dresden pretty well.
For those that haven’t had the pleasure of reading any of the Dresden files yet, Harry Dresden is a PI in Chicago. He also happens to be a wizard. The only Wizard in the yellow pages. There’s a definite Noir feeling to the novels, all in first person. He’s pretty genre savvy too, though he suffers from a mile wide streak of goodness. There’s plenty he just won’t stand for, and damn the consequences. From some of the author forewords in Side Jobs, Jim Butcher is gleeful about making Harry suffer. If you see a happy Harry, it’s probably a setup for a later fall. It’s a world of wizards, vampires (three types!), werewolves, trolls, powerful criminals, underfunded cops, the fae, and pretty much everything else. And everyone keeps mundane people in the dark. Harry gets the weird cases.
There’s been a tv show, which isn’t /too/ bad. but it’s not that faithful to the source material. Read the books first.