There are going to be six books in the Mortal Instruments series. City of Bones, City of Ashes, and City of Glass are the first three. Coming May 8, 2012, is the fifth book, City of Lost Souls. The entire series will wrap up with City of Heavenly Fire in March, 2014.
The Infernal Devices and the Mortal Instruments series are going to alternate in their publication dates — one ID book, then one TMI book, then one ID book, and so forth! Because the stories overlap and intertwine, it should be fun to read them in publication order — though if you choose not to, that will not affect your comprehension of either series. They complement each other, but each stands alone as a separate story.So for the record, here’s the first publication dates for the next six books, in order (THESE ARE WORLD ENGLISH RELEASE DATES FOR THE UK/USA/CANADA/IRELAND/NZ/AUSTRALIA/SOUTH AFRICA/ETC):
December 6, 2011: The Clockwork Prince
May 8, 2012: City of Lost Souls
Clockwork Princess: March 2013
The Shadowhunter’s Codex: November 2013
City of Heavenly Fire: March 2014