I stumbled across a HuffPo interview from March, conducted on the last day of Anomalisa's press cycle, and in it Charlie gives a couple more details about the novel he's writing for Grand Central:

"[...] I'm writing a novel now," Kaufman told HuffPost. "It [is] kind of a way out. Or at least theoretically a way out from this business."

[...]

The upcoming Kaufman novel in his own words “is by design something I think is impossible to make into a movie. Sort of the opposite of what you're usually trying to do with a novel these days. It's like, it cannot be made. And that's what I set out to do."

And what's it about?

"It's about a movie," the writer said. "An impossible movie." (Source)

Also this:

Kaufman is still writing a separate screenplay for "a sprawling satire about the United States," but he's unsure if he'll get the money to make it.

His head will explode if a studio offers to pony up $100 million to adapt the unfilmable novel about the impossible movie.

I wonder if Charlie's read any David Foster Wallace. I'll bet he has.