I like heist books and thieves make interesting main characters. Unfortunately, The Spirit Thief fails on both counts.
Eli Monpress is a wizard who also happens to be a thief. His goal in life is to get his bounty raised to one million gold standards, and to that end he kidnaps the king of a small country where wizards aren't allowed. But his actions open the way for another wizard to come out of hiding, a wizard who doesn't have the scruples Eli does.
My quick synopsis, and the one on the back of the book, make it sound like the book is all about Eli. But after a long section at the beginning where the book doesn't seem to know who the main character really is, it turns out that the viewpoint character is another wizard called Miranda, who has been sent to stop Eli from stealing a valuable artifact. She and Eli end up having to team up against the evil wizard.
In some strange ways, the group of characters reminds me more than a little of the group of characters in the anime Cowboy Bebop. There's even a freaky kid and an intelligent dog (although one that's a bit bigger than a Corgi). But while the similarities are remarkable, the biggest difference is that I like the Cowboy Bebop characters, while The Spirit Thief characters bored me.
Mostly, I never felt any tension. I never believed Eli or his friends were in any real danger. Bad stuff happened to them, and Eli or someone else got them out of trouble. In the end, a deus ex machina saves Eli at the last second, not at all to my surprise. There's precious little character interaction, too, even between Eli and Miranda. They go places, Miranda is surprised that Eli is such a good wizard, and they eventually accomplish precisely what they set out to do. Then they do that all over again.
While I found the characters bland and the plot predictable, I did really like the worldbuilding. The magic system in this world depends on spirits that only wizards can talk to and manipulate. Everything on the planet is a spirit or potential spirit: each creek has its own spirit, which blend into a river spirit, and the ocean is a mad maelstrom of spirits all swirled together--and a waterspout would be yet another spirit, and so forth. It's complicated, well-thought-out, and fascinating. I enjoyed seeing how the different wizards used spirits to help them. It's just too bad the rest of the book didn't engage me as thoroughly.