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I've just started to get intersted in 30k and plan to get into it. Now this is really confusing regarding the books for me. As I understand the book named "The Horus Heresy Legiones Astartes: Age of Darkness Legions" is the book to get for the special rules regarding my wanted legion, ie Death Guard. Is that correct?
Affirmative. That book has the most current version of the Legion specific units and rules, so if you want to run a Legion, it's definitely useful to have.
Now for the other book, well I cant get my head around it. In a previous post here some said do not buy the bok on the right is not a safe buy or confused with another book?
The book addressed in that post was the Legiones Astartes Crusade Army List (often abbreviated to LACAL,) which has recently been replaced with the Legiones Astartes Age of Darkness Army List you mention below. At the time of that post, we knew that a replacement to LACAL was coming, but we had no information beyond that.
There is a book on the homepage named "The Horus Heresy - Legiones Astartes: Age of Darkness Army List", which also comes up if I click the filter to show New Releases. Now is that the correct book to get with the core units and their rules?
Yes, that is the correct book. It's a bit confusing now with the sheer volume of books available at the moment to know what you'll need to run a particular 30k army, but at the present moment it breaks down something like this:
All normal Legions except Thousand Sons and Space Wolves: Legiones Astartes Age of Darkness Army List; Age of Darkness Legions.
Thousand Sons and Space Wolves: Legiones Astartes Age of Darkness Army List; the Horus Heresy Book Seven (Forthcoming.)
Blackshields: Legiones Astartes Age of Darkness Army List; the Horus Heresy Book Six: Retribution.
Shattered Legions: Same books as whatever Legions you are running; the Horus Heresy Book Six: Retribution.
Mechanicum: Mechanicum Taghmata Army List.
Questoris Knights: Crusade Imperialis Army Lists.
Cults and Militia: Crusade Imperialis Army Lists.
Solar Auxilia: Crusade Imperialis Army Lists.
Thanks for answers that will clear this up for me.
You're welcome! Hope it helped!