One thing that I like in a romance novel is a lot of romance: while I want some obstacles for the characters to overcome on their way to happily-ever-after, I also want to spend some time watching the protagonists actually being happy together and in love.
In this respect, Thief truly delivers. There's plenty of intense emotion and lots of tender, touching scenes. If anything, the story is too light on conflict. The book has a large cast of different people, and while they all have distinct personalities, almost all of them get along amiably and civilly. Even when characters representing conflicting sides air their grievances, in most cases they quickly come to accord. It is, on the one hand, delightful to hang out with this assortment of reasonable, well-behaved individuals. On the other, it feels too facile, and the pacing in the second half felt slow due to a lack of urgency.
There were a few other things that niggled at me about the book, but overall I had a splendid time with it. The protagonists are charming. Both of them are composers, and there're some splendid lyrical declarations of feeling that would feel over-the-top in most characters but that fit right. The setting is well-described, with enough detail to convey a good mental image and not so much as to overwhelm. Likewise, I enjoyed the descriptions of the composers at work, which included enough musical details for verisimilitude but not so many as to feel like jargon, accompanied by judicious use of metaphor to convey a sense of the composition. Overall, I'll rate it an 8*, and look forward to reading more in the series.
* A long time ago, someone noted that my actual range of ratings has been 5-9 even though the theoretical range is 1-10, and suggested that therefore my 5 = 1 star and a 9 = 5 stars. But I actually rated this book as 5 stars on Amazon, mostly because Amazon's ratings suffer from the kind of grade inflation that makes 4 stars translate to "meh, it's okay" instead of "this was good!" So I figured I'd try to stick to their implied norms.