There's some lame sub-plot about the Order of the Black Dagger but that's hastily concluded at the end of the book. After bringing Anna out of Hell, the author chose to have Cas suddenly become selfless and release Anna into heaven, even though he knows that the manifestations in there, even that of himself aren't real. This was really predictable. This means that the whole book was about a quest from which Cas would return empty-handed.
I feel the novel would have been stronger if the author had made this a trilogy. The plots were concluded too hastily. You can even see unresolved plot lines. Cas should have brought Anna back into the world, only to realize their selfishness later in the third novel. That would have been more realistic and made a better read.
We often read novels that have characters doing the morally correct thing all the time. This novel would have been stronger if Cas had brought Anna back and faced the consequences of this decision later on. That would have made it stand out from similar novels.
Cas making the decision for Anna was out of character. She barely had dialogue in the book and it would have been interesting to hear what she had to say about moving on.
There was no gore at all in this book unlike in the first book. No new ghostly villains. This book lacked villains. The tone of this book felt very different from that of the first book. I felt kind of sad because the author had a good idea and she just wasted it with this sequel. This book might as well have been called "History of the Athame" because there was so much more backstory given to the athame than to Anna of any of the characters.
The story was really weak. I thought there was a serious danger in Cas and the others contacting Anna when she said they might be overheard. In the end, there was nothing scary and the final battle was an anti-climax.
The new character, Jestine was not significant to the main characters and did not bond with them even at the ending. Yet she received more dialogue than Anna who is the titular character of the series.
You'll be happier reading only the first book and making your own fanfiction about a sequel. I wish I had stopped at the first book.