Spencer’s thesis in An Other Testament: On Typology is, like many powerful ideas, deceptively simple. “This book is about how the Book of Mormon teaches us to read the Book of Mormon” (xix).
The issue at stake here is a perennial bone of contention in Mormon Studies: how should the Book of Mormon be read? Answers vary according to discipline, audience, and temperament, but I’ve never seen anyone else do what Spencer suggests. I’ve never seen anyone else ask: okay, but what does the Book of Mormon itself say about how we should read the Book of Mormon?
I don’t want to spoil the book for you, but the answer is also easy to summarize (see the subtitle): the Book of Mormon teaches us to read the Book of Mormon typologically.
I also won’t try to summarize in this post what it means to read typologically (though I will hint that reading typology is structurally intertwined with repentance). But I will say a word about what it doesn’t mean: the Book of Mormon does not teach us to read the Book of Mormon historically.
Rather, the Book of Mormon insists that historicity is a derivative form of temporality, a form of temporality that is dependent on typological (or messianic) time.
Why? The impulse to privilege historical time is a secular move. It is, in some ways, the secular move. Historical time is secular time. Insisting that the Book of Mormon’s fundamental frame is history is a move that is alien to the Book of Mormon’s own way of thinking and reading.
Does this mean that history doesn’t matter? No, we must love history just as we must love the secular. But the relationship between typology and history is not reciprocal. History is perfectly intelligible (in fact, it’s redeemable) in light of typology, but typology is not similarly intelligible in light of history.
Whenever we privilege history over typology, whenever we assert that historicity is our bedrock issue, typology can then show up, at best, as a weak form of religious fabulation or edifying “metaphor.”
But this, of course, is baloney. The Book of Mormon is no metaphor. And though it’s historicity is manifold (and, thus, complex; and, thus, and non-secular) this historicity has messianic strength only when it is clearly derivative of the typology that gives, structures, and empowers it.
Now, if you’ve got a lot of questions about how this actually works, good. You should.
But this post is intended only as a provocation.
Read the book!
By Common Consent, a Mormon Blog 
Just want to second the motion: read the book! As in, anyone who wants to be in the forefront of thoughtful conversations about the Book of Mormon in the decades to come will need to have read An Other Testament. It’s that important.
It’s fantastic. And thank you for this continued review.