Hi
I had a look at QR codes originally - the size and url links were good - but for the bulk work (like shelf/stock checking) a dedicated bar-code reader wins hands down - like scanning at a check out... beep, beep, beep etc. very quick - with a couple of hundred books, small differences add up.
A QR code on the back/front could work (people reading them will have taken them off the shelf already).
I was thinking of having QR codes on the wall - to take people to the 'check in/out' webpage where they could then scan the actual barcode - a bit of a hybrid system. There seems to be a lot of software for phones to read bar-codes in shops (check for best deals etc) so I don't think phones have a problem with normal codes - its just making the check in/out as simple as possible (without making admin harder!).
Once the software is done (need to sort out the user lookup to log the book to - has anyone got code for that?) we can have a play with different phones/software to see what works - if the first interface is a qr code to a 'check out page' to then scan the actual code I guess the identification will be the browserId on the smartphone browser...
Cheers!
Paul