Thanks to Conks, that's a really useful summary.

I need to catch up, am so far behind, only have book one!

I don't want to wait until 2016 for Ultramarines but in a way this is good for me as it will probably take me until then to paint my 40K Ultras....

I am very intrigued about the Imperial Army aspect mentioned for the battle of Calth book though. Will this be Ultramar Defence Force? Very excited if it is although they'll probably be nothing like mine....