Just like the last book of The Infernal Devices, the last book of The Mortal Instruments (City of Heavenly Fire) will have some extra content inside the book jacket for North American Hard Cover First Editions.
Cassandra Clare has the details:
What the CoHF special first edition cover offers is basically a chance to see Cliff (the cover artist who has done all my covers) do his take on characters that would otherwise not make it onto covers, especially since this is the last book about them.And in another tumblr post, Cassandra Clare tells us:
There are of course two figures on the proper cover of the book, but other characters, some who we’ve never seen on covers before, will be on the inside of the first edition’s jacket.Once again, when the collectors first editions are sold out, that's it -there will be no second batch of collectors editions. Make sure you pre-order the book soon!
And that’s all I can say right now …