3.5 stars. 
 I agree with everyone who says this is NOT a book you want to skip if you're reading the series. It provides so much necessary information. So if you're uncertain about it, consider this another recommendation to pick it up. 
 Maas' writing is really easy to read... I've sort of taken a break from her books for a while and forgotten that. This makes me want to pick up Queen of Shadows and A Court of Mist and Fury sooner rather than later. 
 It's been a while since I've actively wanted to keep reading a book after putting it down, so that was really nice too.