Symbolism, Imagery, Allegory
Books, books and more books: if we had as many books as Rainbows End we'd be classified as a hoarder (and we'd be so happy, because: books!). But the symbolism behind books in this novel is a little trickier than books = hoarder or books = so much happiness.
In Rainbows End books mean different things to different characters, but characters also shift (a little) their ideas about books. Which means it might be hard to finish the sentence, "In Rainbows End, books are a symbol of…"
For instance—
Robert + Books = The Happy Past
At the beginning of the book, we have a clear division between Miri and Robert Gu. If you don't have books in your house, Robert will be able to tell that this is not his house (good job, Rob)—"Not a book in sight; this was no place he had ever lived" (3.10)—and will call you "effectively illiterate" (3.61).
For Robert, "book" means a physical object; throughout Rainbows End, Robert makes the distinction between "real books" (7.14) and those fake virtual books. He even makes the distinction between the physical, printed books he used to have and the new-fangled "just-in-time fake" books (16.67).
That interest in old books and his belief that reading books through the computer is "a tedious desecration" (3.61) helps us figure out why Robert cares about books and what they symbolize for him. But just to push us over the edge into a thesis, here's this:
"I don't care if we're talking high treason!" said Robert. If I can get back my song. ... "I mean, you know what a lover of books I am." (15.67)
We can see a lot in that distance between what he thinks ("If I can get back my song") and what he says he is ("a lover of books"). While Robert talks about loving books, what he really loves is the old way of doing things; for him, real physical books are a reminder of a time when he had his poetic skill and was at the top of his game.
In fact, when Sharif first offers to guide him around the Geisel Library, Robert thinks that it would be like the "old days" (11.22). We're not saying that he doesn't love books. But he seems stuck considering physical books as the only real books because they symbolize a time before he lost his identity as a poet.
Miri + Books = The Miserable Past
On the other end of the spectrum, we have Miri, who grew up in a digital world. When she talks about those physical books that Robert loves, she doesn't call them "real"—she calls them "old" (7.23).
In one of our favorite moments, when talking to Sharif, she says that Robert misses the way books used to be: "those clunky things you have to carry around" (10.72). How much does she sound like an ad for an e-reader right there? But honestly, she's got a point: books are heavy.
Robert (Part 2) + Books = Ebooks Aren't the Devil
Now, by the end of the book, Robert seems to have moved over to something like the Miri/Juan position. He probably still wouldn't like a project that involved shredding books, but he seems to like the virtual world of books a little more. At the end, when he visits the Geisel Library one last time, he watches a student interact with a "book" where the book is made up partly of virtual info and a physical robot:
Somewhere on a high protocol layer, all this involved books and the contents of books. At the physical layer it was even... more... fascinating. (Epilogue.25)
So from thinking that virtual books were "a tedious desecration" (3.61) (the worst type of desecration there is), Robert has come to realize that virtual books are just as real as physical books. Nowhere in that quote does he talk about real vs. fake books.
Now he's interested in the physical technology of robots and computers that make this sort of book possible. So if old books symbolized the way things used to be, these new digital technologies symbolize the way they are—and Robert is finally changing to adapt.