Now, I'll confess, I have not read the book, but for the purposes of this discussion, I don't have to.
We're not discussing the book itzownsef (i.e., how well is it written) but the core idea contained therein, the concept of Universalism, an idea Bell would be first to say is neither original nor unique with him but rather derived from centuries of theologians break dancing with angels.
To wit: If God really loves everybody, why would He even allow a place like Hell to exist, much less allow anyone to spend eternity in it?
Religions that believe in cycles of reincarnation deal with the issue of unrepentant souls & evil actions by saying everybody gets a reset at the end of their (current) terrestrial life.
Behave justly, and you get a power up; collect enough of these and you eventually win the game and enter Nirvana (no, not the band, tho it's safe to say Kurt Cobain is not in either definition of the term).
Behave unjustly, and you go back to the beginning of the game & have to start over as a lesser life form, like a pig or a cockroach or a lawyer (sumbody hep to reincarnation theology lemme know if it's possible to be reincarnated as an amoeba or a bacterium or a virus, because since they reproduce by division they are technically physically immortal, so if a soul gets reincarnated as one how -- and who? -- is it determined when they're done? Or is that the point?).
Classic monotheistic beliefs (Judeo-Christian, Islamic, etc.) hold we only get one shot at the brass ring: Screw up and you are toast (literally!).
Now, there are appeals to both visions of the afterlife. On the one hand, reincarnation holds that there's always a chance to learn how to behave better and eventually achieve Nirvana.
In that view, all souls are already in the Hell of physical existence and all our suffering is just punishment for something we did (but no longer have any memory of) in a previous life.
We escape the Hell of this existence by living justly & alleviating the suffering of others, eventually accumulating enough points so we can quit the game.
Mono-Hell is a far more interesting endgame, full of fire and brimstone, sadistic demons, and screamin' writhin' naked people being tortured for all Eternity.
Boo-yah! Yeah! Those bastards deserve it! Those evil, evil pricks like Hitler and Bin Ladin and Stalin and Dahmer and Gacy and Nixon and my 5th grade teacher and...and...and...
...and even me.
Because ultimately there is only one true sin: The sin of benefiting ourselves at the expense of others.
Of doing to them exactly what we accuse them of doing to us.
In other words, injustice.
 There are points between the two on the scale; f'r instance, Mormons hold that “As man is, God once was; as God is, man may become”, but we'll leave that for a future discussion.
 Which raises the interesting question of how the hell we can learn anything if the reset button keeps getting pressed and all our experience points get wiped, but that, too, is another topic for another day.