Nightvine - The Gardener's Hand, #2

Alizhan spent her life in service to Iriyat ha-Varensi as a mind-reading thief of secrets-until she uncovered Iriyat's crimes. Possessed of both political and magical influence, Iriyat is almost untouchable. Alizhan has stolen a book that might prove Iriyat's guilt, if only someone could read its encoded text. Alizhan and her companion Ev leave the constant sun of their homeland to sail for the Nightward coast.
They pin their hopes on His Highness Prince Ilyr of Nalitzva, a man rumored to understand any language, even one he's never encountered. But Ilyr is not who he claims to be, and danger lurks in the long shadows of the glittering court. Nalitzvan palace guards confiscate the book and throw Ev and Alizhan into prison. Their cellmate, a prickly and mysterious islander, offers to help them break free and steal it back.
They reluctantly join forces and set off on a journey that will test their budding romance and take them farther into the Night as they race to stop Iriyat.