Just finished the 4th and 5th book in the Mortal Instrument Series. City of Fallen Angels and City of Lost Souls.
If this series is one you have not yet picked up, I HIGHLY recommend that you do! It is a hard series to describe without confusing you and I, but the world that Cassandra Clare has created sucks you right in and makes you forget that you have things you should probably be doing instead of reading 24/7. The thoroughness of which she has created this world reminds me a lot of the world J.K. Rowling created with Harry Potter. (Yes everything comes back to Harry with me). I also loved the main character, Clary, and the transformation she goes through and her bravery. Her characters are easy to fall in love with (especially Jace) and that is always a huge part of me loving any book or series. Throughout all the seriousness and the drama there is also a lot of humor that I enjoyed as well, which is also a key to my heart.
If you have read the first three books and were wondering whether or not to continue on (like me) I think you will be pleasantly surprised. After I had read the 3rd book I didn't have a huge desire to continue reading the series right away because it ends so well, no huge cliffhanger. I do love where she has taken the series in the fourth and fifth book and I cannot WAIT to read the 6th. Unfortunately, I will have to wait until 2014. Yep, 2014. I died inside when I saw that. The good news is that they are making a movie of the first book, due out in August. I love that they cast Lily Collins as the main character, Clary Fray, because she is gorgeous and talented. Check out the trailer because it is amazing and will probably make you really want to read the first book! (Or re-read it in my case)
City of Bones Trailer