Where to begin.

I'm conflicted on what to rate this. 3.5 for now rounded up to 4 stars, I think. The concept was great, I enjoyed the story overall, but this had problems. If you read this book without knowing a single thing about who wrote this, you would be able to deduce that it was written by a straight white man.
I will give you some examples:

• “'My husband left me,' she said grimly. 'After fifteen years—a younger woman—I’ll show him!'
'Isn’t it a sin to commit suicide, according to your religion?' he asked. 
She paused, frowning. 'I suppose it is, but—'
'And should you do such a thing to spite him? Why match the wrong he did you with a wrong done to yourself?'
'I am a woman,' she said with a wry smile. 'I owe more to emotion than to logic.'
Zane returned her smile, showing that he appreciated her humor. No woman really thought herself illogical, however strongly she might feel, but it was fashionable to seem otherwise."

• “'It is true. I have much evil to account for. I drove him away. I suppose you know how bitchy a woman can be when she tries.' 
'Not really. I always thought of women as pristine and pure,' Zane admitted. 'Most of the evil resides in men. Women should go to Heaven when they die.'
She laughed bitterly. 'You idiot! There is more sin concealed in women than in men! My husband errs because it is his male nature; I, at least, should have known better. I was fooling myself when I dreamed of Heaven.'”

• "Zane had not before realized how striking a slender woman could be."

• “'I can’t do this,' Lou-Mae complained. 'Singing a hymn to a drum roll?' 
'Listen, black doll, we don’t like it either,' the drummer said. 'But we got to have a beat.'"

Take those how you will. If you can ignore the author's prejudice, the story is quite good. At times it felt a bit juvenile but the concept was strong and it was thought-provoking.
Example:

• "He started to fold it—and the soul disintegrated, falling apart into nothingness. The atheist had his wish. He really had not believed, and so the Afterlife had been unable to hold him. He was beyond the reach of God or Satan. That did seem best. It was best—but was it right? The atheist had not seemed to care about anyone except himself—and in that uncaring, perhaps had rendered his own existence meaningless."

I'm not religious, but it had me thinking and considering ideas of an afterlife and how your beliefs could determine your afterlife. It was interesting to ponder. There is a lot of discussion about religion in this book, some of which may be offensive. 

The plot:
The main character, Zane, finds out that he has a limited time before he will die.

He decides to go out on his own terms and turns a gun on himself only to see Death walk in. Instead Zane turns the gun on Death, ending Death's existence, and thus taking Death's place.

It then follows Zane's "life" as the new Death and learning what exactly that means.

It then becomes a bit of a mystery quest to find out what Satan's grand plan is, how to stop it, and what it means. I would say it's an intriguing read and worth it, but as stated above, there are many issues with how this author chose to write his story.