The Assassin's Blade

It's no secret that "Throne of Glass" must be my favourite series EVER. So "The Assassin's Blade" could not be bad. And it isn't.
This book contains five novellas, all set before the beginning of "Throne of Glass" itself. The first story ("The Assassin and the Pirate Lord") starts about a year before, is then followed by "The Assassin and the Healer" (which is shorter than the other four, due to the fact it was originally a UK World Book Day Novella), then comes "The Assassin and the Desert", "The Underworld" and, finally, "The Empire". Those five stories in fact tell how Celaena found her way to the Salt Mines of Endovier, where we find her at the beginning of the first book.
The story explains a lot of strange remarks, notes and thoughts you get from Celaena.
If you have read the books in the main series, this is a must-read. And if you haven't, it's still a must-read.