“For last year’s words belong to last year’s language,” T.S. Eliot wrote, “and next year’s words await another voice.” It would be hard to find a more apt description of Joshua Cohen and his brilliantly exhausting fourth novel, “Book of Numbers.” On its surface, the book is about a struggling New York novelist hired as an Internet billionaire’s ghostwriter — but its breadth, the ambition of its ideas and devices, confounds standard book review responses. Trying to approach this demanding, overstuffed novel is a bit like hyperlinking one’s way around the Internet: It’s bigger, wilder and fuller than you imagined, and there’s always more where that came from.