So if you check out our discussion of The Trials of Arabella, you'll see we mentioned self-referentiality. In case you haven't checked that part out yet, though, self-referentiality is when a book talks about itself. And how better for a book to self-reference itself than by using books to symbolize the book you're reading? It's a book about a book that talks a whole lot about books.
There are lots of books plumped down hither and yon in that book you've got there called Atonement. You'll find them especially in the vicinity of Robbie. When we see him alone in his room, he's got tons of books scattered about, covering everything from poetry to medicine. Then he picks up the book on landscape gardening he got from Cecilia, "raise[s] the book to his nostrils and inhaled" (1.8.11). A couple of paragraphs later he mentions "the page at which his Anatomy tends to fall open these days"—that page would be the page illustrating female genitalia in case you weren't sure. At the same time as he thinks about his beloved Anatomy, Robbie writes at the end of his letter to Cecilia, "In my dreams... I make love to you all day long" (1.8.15), amongst other even racier things.
For Robbie, Cecilia and books and dreams all run together. This makes sense, since Robbie is himself in a book within a book—Ian McEwan writing about Briony writing about Robbie and Cecilia. No wonder, then, that when Robbie and Cecilia finally come together it's in the library, their story torridly shelved among the stacks of books surrounding them.
After Robbie is sent to prison, he and Cecilia turn to books to express their love and desire for each other in code to elude censors. "So they wrote about literature, and used characters as codes" (2.80), mentioning great literary lovers to stand in for themselves.
And then, "Mention of 'a quiet corner in a library' was a code for sexual ecstasy." After Cecilia and Robbie's romantic rendezvous in the library, libraries (which, you know, are filled with books) become themselves a symbol for love.
So why the obsession with books? Why a novel about novels? Well, we talk about that some in the "Why Should I Care?" section, but maybe the short answer is just that novelists are pretty into books.