Having read the first Dresden book - Storm Front - and having seen the TV show for it's brief run, I am familiar with the setting/characters in the "Dresdenverse", as someone so aptly called it.
The best I can say about this little novella is that if you are a fan of the series, you'll enjoy it for the different angle on an event you're already familiar with. If you're collecting the series, it *might* even be worth the exorbitant cover price to own it.
For everyone else - borrow it from the library, or a friend.
Actually, if you're not familiar with the Dresdenverse, don't bother untill you've read some of the main books. The entire thing is written like someone confiding in someone who is already 'in the know', but the 1st person voice is kind of foolish, I thought, since the main character tells us over and over again that he can't tell anyone this information. It left me wondering, then, just why he was telling me? He's not rambling on to himself, he's actively speaking to another, explaining all sorts of things.
Ultimately, this is the jarring detail that ruined the story for me. Of course, there is also the skipping-all-the-details of things happening, which is a big story-killer for me, but ultimately I just couldn't get past trying to figure out just WHO was the narrator talking to, within the severe confines he had set up around the direst need to NOT tell anyone about the "Oblivion War", even the name of it.
Oh, well. Enjoy it if it's your 'thing' and you can overlook the obvious flaws, but to me it's just bad story-telling.