Ghoulfriends 'til the End (Monster High #4) by Gitty Daneshvari
Well, the words "The End" make this look like this is the end of the series. Or maybe it's just the end of this plotline. I know everything will work out all right; it's a middle-grade kids' book, after all, but how the ghouls will get from where they are to the happy ending will be the interesting part.
This series is still awfully cutesy, but this book was more complicated than I expected. "Ghoulfriends to the End" opens at Crack and Shield Day (as always, I take no responsibility for that one). The ghouls corner Wydowna and try to convince her to tell them about ASOME, the secret society that the Ghouls discovered in the previous book. Just as she is about to give in, they are interrupted by a call for the students to return to the building. They lose Wydowna in the crush and when the students of Monster High count off, one is missing. Wydowna is not among them.
The plot thickens once Miss Flapper broaches the idea of approaching ASOME to help the monsters of Salem repel the normies. The ghouls, of course, don't believe that the normies are any threat and, rather, believe that ASOME itself is the threat.
We find out what the list that the ghouls found in Wydowna's room is and what that list means for the future of both Monster High and Salem.
There is, unfortunately, a little too much "we know what's best for Monster High" from the ghouls as they go sneaking around to get the information they (and, of course, we) need to figure out what is going on and how to prevent the incursion of ASOME into their school. Fortunately at least one adult, Miss Sue Nami, agrees with them about ASOME. However, she thinks that the ghouls should stay out of it.
Also, I get the feeling this book was rushed to print, since there are a few typos, including one place where Fanglish is called "English." Perhaps future editions (including the paperback version) will fix these problems. One can hope so, at any rate.
And I was kind of right in my introduction. "Ghoulfriends 'til the End" is the end of this storyline, but not the end of the series. "Ghoulfriends 'til the End" ends with an epilogue that sets up the next storyline in the series. I really enjoyed this plotline and will likely read the next one as well (though whether I will buy the books or just check them out of the library remains to be seen).
Update (November 17, 2016): Maybe this was the end of the series. It's been two years since "Ghoulfriends 'Til the End" was published and I can't find even the slightest rumor that there will be more books in this series.