Siege of Terra out of order?

The seventh of the eight books in the Siege of Terra finale for the Horus Heresy saga will be with us in the next month or two. We recently saw a preview cover for another ‘in between’ novel – not one of the numbered books, but designed to fit between them.

We previously had Sons of the Selenar, which fit between the first two books, and Fury of Magnus, which fit between books 4 and 5.

This book is a bit different:

Nathaniel Garro has been a recurring character throughout the series, and this book promises a conclusive battle between Garro and his former Primarch, the now-daemonic Mortarion.

Thing is, from the description this looks like it would naturally fit between books 5 and 6. Anyone who has read book 6 (Warhawk), probably already knows how this one will go.

The vagaries of publishing dates aren’t always on our side, and it looks like this book should have been released last year, not this year. I’ll still read it when it comes out though.