In reading Evil at Heart I started to wonder what are the things that make a series with a multi-arc book work, and where is the point where the reader gives in.
I may be spoiled by romantic suspense where if the killer isn't caught at least something clear is resolved, and I felt Evil at Heart answered some questions, but I did ponder how many books I would be willing to wait to see something of this nature resolved. (I have, in contrast, been burned by series where one thematic element appeared to be dragged ever further, and there were whole books where nothing much happened. That is clearly not the case here. But I pondered.)
In contrast, I have enjoyed Rachel Vincent's were-cat series (note, in contrast because I have not sat to ponder how long I would wait for stuff to get wrapped up, the enjoyment part is not the contrast for me). Some readers have suggested that some of the books end on cliffhanger's as the bits are thrown out for the next book. So far (Vincent has suggested that there is one that ends on a cliffhanger coming) these have not bothered me. It helps that I follow Vincent's blog and knew it was part of a series, and know about how many books are left.
It can be comparable to television also, where the over arching mystery can drag on, but individual episodes may focus on smaller issues in order to provide some temporary distraction. Now, I am not picking on Chelsea Cain. I have certainly read books that did things that to me were egregious such as ending on a question. (I have seen that twice and in both cases I refused to read anything else by that person.)
Cain has said that to her, the book's focus is the relationship that develops between Archie (the leader, at least initially, of the task force, who later becomes a victim himself, although an alive one) and Gretchen (the killer). In that sense it is a bit different from a traditional serial killer novel in that, now at least, the killer is known not just to the audience but to the characters. This allows Cain to focus on things like the relationship between the cop and criminal, look at media portrayal and many other things, not the least of which is the idea that Gretchen has been dubbed the beauty killer becaase she is pretty.
So, in that sense the book provides exactly what it sets out to provide.