The 14th History of Middle Earth Book?

[Hookbill the Goomba]

Whilst on a brows of the Amazon website I came across this. Now, I recently completed my collection of all 13 Books in The History of Middle Earth series (Index included). This Book, “The End of the Third Age”, is not one of them. It says it is part of The History of The Lord of the Rings series, but so too are "The Return of the Shadow", "The Treason of Isenguard", "The War of the Ring" and "Sauron Defeated", All of which are numbered in The History of Middle Earth series.
Unless this book is being numbered 13 (The Index is not numbered in the UK editions) or 9.5 I cannot see where it will fit.
I have now ordered the Book in question and was wondering if anyone had it and could tell me if it is counted among The History of Middle Earth series. As it has the same design as the others, it looks as if it will fit.

Any information would be greatly appreciated.