Oh those Wachowskis, they just can't help stuff their movie full of philosophy. Even simple hollowed out books used to hide digital contraband are meaningful.
When Neo goes to get the goods for Choi and his gang, he opens up a book called Simulacra and Simulation. The book is hollowed out and inside Neo keeps all his illegal goodies. The book itself is a philosophical treatise by Jean Baudrillard. In it, he talks about signs and symbols that humans use in place of reality. This means that, in a sense, they are more real than the real—they are hyperreal.
That probably sounds somewhere between confusing and nonsensical, but it actually makes a lot of sense. These signs can be anything—think of language for instance. Every word we speak or write is a symbol of something that has some type of physical existence, whether it's a noun or verb or adjective. Our word "cow" is not just a few weird squiggles on a screen or piece of paper—it's a few weird squiggles that make us think of a weird mooing beast.
It's probably not hard to see how this applies to the Matrix, but we'll give you a fun example that Baudrillard gives in his book anyway. Pulling from a short story by Jorge Luis Borges who was drawing from a chapter of Lewis Carol's Sylvie and Bruno Concluded (wheels within wheels, we know) Baudrillard talks about a map so detailed that it must have a 1:1 scale to the land it represents.
When the Empire that the map represented finally decays, the map is the only thing that remains. Its existence is more real than the land itself. This is what he calls the "desert of the real itself" and Morpheus repeats this phrase as he shows Neo the desolate "real" world and it's stark contrast to the allegorical "map" the Matrix.