Queen of the World was very charming and enjoyable and easily held my attention. Hennessy is an adept, natural writer. The characters -- especially Sarene and her gruff and loyal guardian Kanderil -- are people you want to be with, and they have strong, believable arcs. I don't want to give anything away, but the monster assassin Spasmodic is a fun character, built for violence and wielding a smart mouth.
The Four, a corrupt, all-powerful faction who have not been seen for five hundred years, are a daunting foe. They're after Sarene because she -- a mute, pure-hearted girl -- presents a major threat to them. There are various bands of assassins after Sarene, a young girl who has relied on her brother and Kanderil to fight on her behalf. She acts admirably leading up to her act three resolve to stop hiding behind others. Kanderil is a near-giant of a man and no fan of bustling towns. He has an intriguing past, and is a formidable protector. "composure was one of his strong points."
There are touches of humor throughout, mostly from Spasmodic (who speaks in British slang), but also from other characters, e.g.:
One arm came to his first chin, tapping his finger against the firmest point. "If I *were* to have a tunnel, then where would I put it…"
"If you're tricking me," Jared said, "I'll send you back to the pit you came from."
"Actually came from a table," replied Spasmodic.
"Even so, I dislike having vital information concerning Sarene's whereabouts tied to a pigeon."
I enjoyed the descriptions of the homey interiors and of life on the road. Hennessy has a lot of nice worldbuilding detail and sharp descriptions that really make this setting seem real, e.g.:
The two horses had been unharnessed and were creeping at the ground with broad, flat teeth, hunting the slim pickings of grass.
A few of the character names were a bit too similar to the group they aligned with (or used to align with) and there was a brief part in the last quarter of the book involving ancillary characters where I didn't feel as connected, but maybe a second read would be different. Also, my ebook version had some minor formatting problems, which the publisher fixed in an update, though it wasn't bothersome enough for me to get it. Overall, this book was very enjoyable and well-written, with strong characters. I look forward to the continuance of this as a series, and highly recommend it for adults and young adults.