Good books to get you up to speed:
Cadia Stands - to see what happens to Cadia
The Emperor's Legion (Volume 1) (Watchers of the Throne) and it's sequel, shows what a mess Earth is and the mobilisation of the Custodians including the return of Roboute Guiliman. Also they are written by Chris Wraight who is one of the best authors they have at the moment.
Then you could jump straight into the Dawn of Fire books, as they detail the start of the Indomnitus Crusade setting off from Earth.
The Dark Imperium books are a bit hit a miss IMO, and I didn't hear good things about the Gathering Storm series but they do detail a lot around the Eldar involvement.