It can be hard to collect DWJ's short stories without buying a lot of different anthologies. This book brings together 15 of her short works, plus the hard-to-find novella Everard's Ride. Many of the the stories were originally found in Warlock at the Wheel and Minor Arcana, so if you own either of those books, you will find some duplication. However, no Chestomanci stories are included in this book, so no overlap there.
I personally think DWJ's novels are better than her short stories, with a few exceptions. But I still wouldn't want to be without them, and this collection is worth seeking out if only to obtain Everard's Ride. For a new reader, though, I would recommend Charmed Life or Howl's Moving Castle.
I'm not sure why this book isn't published in a British edition; as a result, the spelling is Americanized. A small quibble, however, with what is overall a valuable and enjoyable book.