Unleash the storm is the last book following Ash and Piper. 
 I liked Unleash the storm but it was left a little open ended. I'm not totally sure how their personal story will finish. We are just given a vague idea on how their life will proceed in the immediate future. I think that is because we will probably see them again in the new series (focused on Lyre, I think). However, since I may not read that series it was a little annoying. 
 (view spoiler)[ I would have liked to know if Ash and Piper stay together or if they eventually have to go separate ways and make separate lives for themselves permanently.(view spoiler)[ (hide spoiler)] (hide spoiler)]