36 Arguments for the Existence of Godactually is a work of fiction with characters and a plot and the whole nine yards.
To be clear, it's not "a novel," it's "a work of fiction." And you have to believe that someone as intelligent as Guggenheim fellow and Harvard faculty member Rebecca Newberger Goldstein would choose her words carefully when deciding how to describe her book on its cover.
My guess is that she is hoping to subtly signal to her reader not to expect a novel, as one would normally conceive it. Instead, because the characters on which Goldstein's builds her work of fiction are almost too easily recognizable, too typical, it's pretty clear they are just vessels. The real point of this work of fiction isn't the fiction, it's the work required to understand the ideas. That's not inherently a bad thing, just something to be aware of.
Cass Seltzer is our protagonist — a middle aged college professor, and best-selling author of an atheist tome titled The Varieties of Religious Illusion. Despite his fame (notoriety?) as an atheist, Cass's charisma and humbleness have earned him the label "atheist with a soul." Cass is brilliant, but for all his logical faculties, he can't quite seem to reason out love. His first marriage ended when his wife got sick, and then fell for her doctor. Now he's dating a fellow academic, and he's trying to decide if he loves her. "Romantic infatuation can be form of religious delusion, too," Cass realizes at one point.
Then, there's Azarya, the child genius "imprisoned" in that Hasidic sect. The kid is a math prodigy, proving that there's no largest prime number at the age of 6. Azarya's purpose in the book seems to be to give Goldstein a vehicle for discussing the philosophy and ethics behind "wasted genius." Will Azarya waste away in the insulated religious sect or will he be permitted to leave and enrich the world with his gift?
Much of this work of fiction is told as conversation, or, like the first scene of the novel, Cass standing on a bridge in Boston doing an internal review of his current state of affairs, much is also told through characters' contemplations. There is also quite a bit of description of Jewish mysticism and Hasidic ritual, much of which is a real slog, frankly. The novel is capped off by a Harvard debate between Cass and another guy about the proposition "God exists." Who will win?
It should be pretty clear by now that this novel isn't exactly beach reading. I'm not going to lie, it's hard work at times to keep up with the arguments and concepts. And so to use a cliche, you get out of it what you put into it. I was a lazy reader on this one and didn't expend the necessary effort to really enjoy it. So I didn't. But you might...
Tip on reading the work of fiction, should you decide to: The Appendix contains the 36 Arguments for the existence of God Cass included in the appendix of his book. There are (surely not coincidentally) 36 chapters in Goldstein's book, so I read one argument in the Appendix after each chapter. That seemed to work out nicely, and made them more interesting and manageable. I can't imagine reading all 50 pages of those 36 Arguments after finishing the story.