The Visitor (The Graveyard Queen Book 4) by Amanda Stevens My Opinion-This one isn't as dark as the first three, but it is just as mysteries. There is a creepy factor and a new mystery is born. We begin to see the heartache of Devlin pulling away from Amelia. Amelia gets stronger in her abilities. 'The Sinner' is the next book. https://www.amazon.com/dp/B0166B9IAE/ref=cm_sw_r_pi_dp_2ePLxbNZWGN1T
despite the title, this book is actually incredibly thought provoking and better described by the author as being about "the horror of being alive in a world where atrocities happen". worth the read if you've got the stomach.
The Kingdom (The Graveyard Queen Book 2) by Amanda Stevens My Opinion-We don't see Devlin much in this book. We meet a new character Thane who is a mystery & possible threat. Some of the questions Amelia background are answered & of course others are introduced. Its a little lighter then 'The Restorer', but there is still darkness & mystery. The next book 'The Prophet'. https://www.amazon.com/dp/B006YAD7P0/ref=cm_sw_r_pi_dp_LcPLxbXHNNWJX