Short summary:

Xanadu comes to visit Cassandra because she senses something very dark there.
Stranger and Doctor are taking care of Zauriel who is getting worse and worse. They notice another crack (like in last issue) at Cassandra's place and Stranger leaves.
Some force field is blocking entrance, but Stranger and Xanadu manage to come in. Xanadu refers to Stranger as a friend which he finds unexpected, safe to say that they have some old history.
They notice that all books are gone from the shop and when they enter next room they see possessed Cassandra with all the books flying around.
Basically she had lots of old magic books and all the spells managed to form some kinda identity on their own due to crack. Weird stuff as is kinda usual for this series.
As per standard Stranger gets knocked back on his ass.
We got some exposition from Sin Eater and his master, black dog called Non. Non with the help of Sin Eater is creating these cracks that eventually should end up with Heaven and Earth colliding and Non taking over. Sin Eater is kinda like his general whom he gave free will (everyone else in his army is basically undead zombie), as expected Sin Eater just wants to kill Stranger, doesn't care very much about invading and ruling stuff.
Anyway Stranger and Xanadu have to bail because possessed Cassandra is too much for them (playing with kid gloves again, just like in Blight). They leave the shop, shop starts flying towards the crack in the sky.
Realizing that something bad might happen if shop reaches the crack they follow it and Xanadu tries to kill Cassandra because they can't risk it. Stranger at first tries to stop then admits that its probably the best option now.
I think Zauriel dies, final words "Tell him that I always lo--". He always loved Stranger?
As per usual Stranger starts flip flopping and slaps Xanadu away saving possessed Cassandra. Cassandra is glad and comes near him, Stranger goes for the kiss and sucks up all the bad spells into himself and disperses them. This closes the crack.
Apparently that was the plan, Xanadu pretended that she wanted to kill possessed Cassandra and Stranger stoped her so that Cassandra would let her guard down.
Spectre allows, as per usual some insults are thrown around. Basically heaven and earth are about to collide and only Spectre and Stranger can stop it.