It can't be helped, comparing this novel to Erin Morgenstern's grand-slam debut, The Night Circus. But, even with many, many years of work in-between novels, this one lacked the magic.
In her earlier work, there were many characters which the readers would either identify with, secretly long to be, or secretly long to be with. Each of the characters were introduced to us, fleshed out, and we already had a stake in them when they decided to, for reasons of love of others, do the things they did in that novel. (No spoilers here...)
In The Starless Sea, everything we will ever know about the main character, Zachary, is told to us in a couple of sentences early on: he's a gay computer gamer with literary interests. Introducing his orientation allows the author to have him fall in love with the first guy he sees, who, unlike her previous effort again, we haven't had the time to get to know or love him ourselves. The fact that his mom is a fortune teller is repeated often, but we don't spend time with that character until near the end of the book, and she adds little to the story.
None of the characters resonate with the reader, and the best parts of story are too short.
At the end of The Night Circus, I was left wanting more, the future adventures of Bailey, of Celia, of the twins -- they all were memorable. The only character in Starless Sea I'd wish to learn more about is the cat.
It wasn't a horrible book, but pales in comparison to her earlier prose, and came out as just plain "decent". Borrow from the library instead of buying -- it has no re-reading power.