Judging from his latest insult to trees, Dan Brown has apparently tried to read Dante, and yet his sentences are still stacked like so many corpses.Source
What does the writer mean here?
Trees are chopped down and turned into paper so that his books can be printed. In other words, trees are being turned into Dan Brown novels.
In this reviewer's opinion, the books are so terrible that doing so is an insult to trees.
As snailboat mentions, the reviewer regards Dan Brown's novels as an insult to trees (which gave their lives that the books might be printed).
The phrase “sentences are still stacked like so many corpses” suggests that the writing is dead and lifeless.