The highest density found in commercially available barcode symbologies appears to be PDF417:
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PDF417 - Known as a 2D (two-dimensional) barcode, this is a high-density, non-linear symbology that reminds you of a crossword puzzle. But the difference between this and the other barcodes listed above is that PDF417 is really a portable data file (PDF) as opposed to simply being a reference number. Some states require a 2D barcode be printed your driver's license. If your state has this requirement, it's interesting to know that there's room enough in this barcode to encode your name, photo and summary of your driving record, and other pertinent information. As a matter of fact, a PDF417 barcode can encode the Gettysburg Address in a space the size of a postage stamp!
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This means that a book would still take up considerable space. It is not beyond the realm of possibility however. I think it was Byte magazine that offered bar coded software every month in their publication. There are experimental symbologies that have higher densities. For bar codes see
here and
here.
RFID is a passive technology that can be embedded in a book cover. There are powered tags and unpowered tags. At this time the applications envisioned for this technology do not support the kind of information density we would need for a book. So far, they have been used in libraries to identify books. Information capacities are expressed in bits, not bytes or megabytes. Densities of 64 bits and 192 bits are typical.
Smart card solutions require a physical connection. The smart card contains a microprocessor, typically a PIC, I think. Theoretically this technology could be adapted to a higher memory density sufficient to encode a book-length file. The interface protocol would, in all likelihood, no longer be smart card compatible, but a standard unto itself. Power for the circuit is provided by the reader, so batteries are not an issue. This would be no cheaper than an SD card.
Perhaps the simplest solution would be to incorporate a mini or micro SD card under a plastic flap inside the book. It could be sealed, so the buyer would know if it had been tampered with, in the same manner as a CD in the back of a software manual. The SD card solution has the advantage of being readable by most readers. Of course it would contain the book in multiple formats. Could a low-capacity SD card be included at hardback prices?
The cheapest solution would be the tried and true CD in the back of the book. This would influence book size (trade paper back?) and require a PC to load the book onto the reader. Not so good for travelers, but easy for the industry to adopt.