Eclipse of Hope
I loved it! The first Blood Angels Omnibus was the very first 40K thing I ever read. Never played the game, so knew absolutely nothing about this stuff. (I'm sure it would be a very different read now, 25+ books later.) I was very impressed with Mephiston (among other things, particularly Rafen), and started looking for psyker stuff. Well, this is a 1st person pov Mephiston story, and he rocks!
I believe it's also set earlier than the Omnibus. I don't know what year Calistarius became Mephiston, but EoH seems to be relatively soon after that.
It also seems clear that there will be a sequel.