I both adored The Emperor's Soul and was frustrated by this book at the same time.
The story revolves around Shai, who has been captured after forging the Emperor's Moon Scepter. "Forging" in this book isn't forging in the sense that you and I understand it in this world (uh, that is to say, the real world). Instead, it's a magic-based process that involves copying, creating, or changing any item by rewriting its history. (It's a bit hard to explain in a synopsis-like fashion, as it's a bit complex, but it's also really cool.) Shai faces execution for her crimes, but is allowed to live under the condition that she forges a new soul for the Emperor, who is alive after an assassination attempt, but lives in a vegetable-like state.
The Emperor's Soul takes place in the same world as Elantris, although honestly, if I hadn't known that before reading it, I might have never made the connection. There are none of the same characters, and the empire/locations in The Emperor's Soul are different than those found in Elantris (although I remember a very brief mention of Svorden and Jindo). The closest connection is the magical system, because like AonDor and Dakhor in Elantris, the Forging in The Emperor's Soul involves using a writing-based technique in order to evoke some sort of extra-normal event.
Although The Emperor's Soul was absolutely beautiful for what it was, I felt that it could have been on another level entirely had it been fleshed out more. Now, keep in mind that this book is only about 175 pages, so it's a short book. It's amazing what Sanderson was able to accomplish given the length of The Emperor's Soul - the story was (more than) aptly told, and the message was powerfully delivered. But yet, there were just some critical things that I felt were missing.
For instance, the books centers around Shai, and we get to know her well by being privy to her thoughts and feelings throughout the story. We never learn much about her past, however, and this vexed me because I was so, so curious! There is one brief scene in which a tiny bit of her past is revealed, and I found it completely fascinating. It just wasn't enough for me. This is just one example, but there's so much more that I wanted to know that I was never able to learn.
With that said, there are a lot of things that I loved about this book. The highlight, for me, was the development of the relationship (*not* romantic) between Shai and Gaotona. I don't want to go into detail on this for spoilerific reasons, but it was just well done and poignant. The nature of Forging was also really cool, and Sanderson used great literary imagery to bring this magic system to life. Finally, the story had the kind of ending that makes you want to take a moment and stop what you're doing, so that you can soak it up and let it ruminate. I don't know if the book's message was particularly strong - at least, it didn't resonate with me, personally - but it certainly felt strong at the time it was delivered.
This also seemed a bit unique in style to the other Sanderson works I've read (Elantris and the Mistborn series), so it was nice to see Sanderson show some versatility.
All in all, this is a wonderful read, but it fell a little short of perfect due to some missing details and it's lack of overall staying power.