Games Workshop has a tendency to sometimes post rules for similar armies, or new units for similar armies, across a wide range of publications. As such, it can sometimes be tough trying to play those armies, as you've gotta carry around a bunch of books and flip through all those pages to find the rule you might need. It can be rough. Thankfully, they will occasionally put out a compilation book that brings together all those rules in one place. The Horus Heresy Legiones Astartes: Age of Darkness Army List book is just such a tome, and you can order yours now.
So, what all is in there? Well, that would be all the various Space Marine Legion units, from command to troops to support, that use the Battles in the Age of Darkness expansion for 40k. There's a total of 65 units inside. The book also includes rules for various fortifications. There's even rules for things like the Stormbird, as well as the Legion Jetbike Sky Slayer Support Squadron (try and say that 3 times fast).
There's also a special deal going on. If you order this book as well as the Horus Heresy Book One - Betrayal book at the same time, you can get the second book for just £64. That special lasts until the end of the month.