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I refuse to write a review that spoils a book for any that haven't read it before and I think there is nothing worse than reading a review on a later book which is an essay long of back story. So I am going to be concise but give you enough of my opinion to hopefully help in your decision to buy / read this 4th installment.
I'm a fan. I think Cassandra Clare as a story teller draws you in and keeps you there. With the first three books in the Mortal Instruments Saga you couldn't help but fall in love with the characters and the flowing way in which the plot unfolded. At the end of Book 3 you wanted more but the story did sum up quite well... which I think is why City of Fallen Angels almost has a different skew to the rest.
The first thing that I noticed when I started this book is that, while it is roughly the same number of pages, the font is larger and the margins more pronounced; clearly not that same amount of story. I felt a little cheated but carried on and felt my disappointment grow slightly. Both the plot and the interactions between the characters felt lightweight and lacking the same amount of depth as I had previously loved. Chapters seemed flighty, you were just getting into a scene when you would be flipped to another storyline involving completely new characters. To me it felt like this book was written as a demand exercise, new plots developed just to keep the characters going.
But... it did keep my attention. Loved characters developed where they might have become two dimensional. There were twists and turns which I didn't expect but fit well. Threads which had been briefly detailed were now woven more deeply into the plot which was great in the sense that loose ends that had intrigued me had me thinking 'oooh so that's why!' Even though I am older than the typical YA fiction reader this book did get me excited enough to go hunting out the 5th book even at the half way point.
In summary CoFA is well written, well thought out and a good edition to the saga even if not the best of the bunch.