Have you read Neil Gaiman? If so and you enjoyed him... then by all means pick up this book and relish it. If you are not a fan of his longer works like American Gods, then leave this on the shelf.
And if you are one of the few out there who has never read Neil Gaiman, may I say that you should prepare yourself to be highly entertained.
Don't let the tittle fool you. Yes, there are dark things (secrets, monsters, shadows) lurking in the pages of this book... but they are more interesting than scandalous, more intriguing than horrible, and more enjoyable than you might otherwise think.
And a few of them are poems.
Like many short story collections, the theme here is rather loose. I would challenge you to ignore the story by story explanation of how and why each entry was created until after you had read them and restrain yourself from playing the game of "where is the dark thing?" while reading each piece. Your enjoyment will go much further if you simply relax and enjoy the ride through poetry, fantasy, horror, and genre stories that all beautifully well crafted.
I personally found the retelling of a familiar fairy tale and the return to already known fandom universes to be the more enjoyable parts... but my favorite story is one of the longer ones that could have very easily been lengthened into a high fantasy story all its own but works so much better as a short sharp addition in this format.
Do not read these short stories looking for happy ever afters, profound epiphanies, or endings with bows. Read these for the adventure and the challenge. Read them for the mystery and the fun.
But please, read them.
I highly recommend this short story collection.