This is Book Three in The Mortal Instruments series.
This book feels like the culmination of a trilogy. It contains a mortal war of Shadowhunters, Downworlders, and Demons. Various romantic problems are solved. All is right with the world...but there is Book Four waiting at home for me to start. I don't know how I feel about this. I like my YA to end happily and my series to max out at 3 books (with the exceptions of Harry Potter and Outlander, which could go on forever, in my opinion).
I really do like this series, so I'm afraid I'm just going to have to keep reading...no matter how many books she writes...but I don't think I'll read the prequel series because I just can't get attached to another series.