City of Lost souls begins almost cheekily. Jace is missing, Clary is determined to find him, but is offered no help from the Clave (aside from letting her off that pesky charge for raising a certain someone from the dead.)
The cheeky part is Clare almost makes City of Lost Souls seem like simple lost lover story. Instead, as the book progresses the twists turn into betrayals, the desperation of the searchers intensifies to the point of suicidality, and the humour... well the humour is a little random, if not exactly what is needed to prevent this series becoming a supernatural whine-fest.
The main problem with this story is possession seems a recurring theme, and an awful amount of the tension appears to be resolved through appealing to better natures (whether its an angel, lover, or mute brother.)
That and the pacing seems a little whack - as the book should be soaring towards a dramatic ending, we suddenly get some of the more boring characters resolving their story-lines, also the epilogue was way too long and had some plot points I confess not understanding completely.
Given what seems to be a generally negative reaction, I'm going to hazard a 'this won't change your mind' opinion. However if you're still way into the series than this one won't hurt either.