Mask of Shadows by Linsey Miller

Sal is a thief.  At the moment, she's a highway man and she's hoarding bits of her stolen goods to try to leave this life and go on to something better.  Her boss isn't easy to get away from.  When she finds out they are going to have auditions for a vacant position at the castle she decides it's time.  The Queen needs to find a new assassin for her Left Hand.  They are her hired killers and Sal wants to avenge the deaths of her people, especially her own family.  She begins practicing by killing her boss and taking his hand along to prove it and gain her entry.

Sourcebooks Fire and Net Galley let me read this book for review (thank you).  It will be published August 29th.

She gets accepted.  From now on she's named as a number:  Twenty-three.  There are rules.  They have classes.  And they are allowed to kill the competitors...

Sal walks a fine line here.  She's not a natural killer but she has people she wants to kill.  She has to protect herself and her maid.  And she's got to stay alive to become Opal of the Queen's Left Hand.

Sal is gender fluid and plays the part of male or female depending on the clothing she wears.  She most often acts male but she's not big bodied so others assume she's female.  They make other assumptions which cost them.  Some even pay with their lives.

Sal falls for the lady she robbed in a carriage in the past.  She's her instructor in this new life and they spend enough time together to flirt and carry on a romance.

This story focuses on Sal's quest to become an assassin and to kill those who left her people stranded with the shadows who destroyed them.  She manages to do her assignments but things are still going wrong.  The Queen isn't who she thought she was, the assassins do what they are told with no questions and this is hard for her, and her love gets kidnapped by her father.  At the end of this story, the tale is just really beginning.  It will be interesting to see what the next book is about.