This is the second novel I've read by Hoang (the first was The Kiss Quotient). Both are romantic comedies where one of the parties has autism. In this case, Khai (who is the cousin of one of the main characters in the prior novel) has autism and he believes it means that he is incapable of feelings, including grief and love. His mother thinks otherwise and travels to her native Vietnam to find him a wife. There she finds Esme, who as a mixed race single mother who never knew her father has never really felt like she fit in or was "good enough".
Khai's mother convinces her to travel to California and live with Khai to try to get him to marry her - not as a trick, Khai is aware of the plan just expects to be resistant to it.
On these facts a fairly normal rom/com ensues. Both parties fall for the other; both believe the other could never fall for them...etc. And then of course there is a happy ever after.
As an aside, while it added some dimension to recognize the characters from the prior novel, it is by no means necessary to read it first.