OpenURL is the nifty protocol that libraries use to find the closest copy of a electronic resources and direct patrons to copies that the library might have already licensed from commercial parties. It's all about finding the version of a resource that is most accessible to the user, dynamically.
Say I've loaded 500 eBooks into my eBook reader: a couple of encyclopedias and dictionaries; a stack of books I was meant to read in school but only skimmed and have been meaning to get back to; current block-busters; guidebooks to the half-dozen countries I'm planning on visiting over the next couple of years; classics I've always meant to read (Tolstoy, Chaucer, Cervantes, Plato, Descartes, Nietzsche); and local writers (Baxter, Duff, Ihimaera, Hulme, ...). My eBooks by Nietzsche are going to refer to books by Descartes and Plato; my eBooks by Descartes are going to refer to books by Plato; my encyclopaedias are going to refer to pretty much everything; most of the works in translation are going to contain terms which I'm going to need help with (help which theencyclopedias and dictionaries can provide).
Ask yourself, though, whether you'd want to flick between works on the current generation of readers---very painful, since these devices are not designed for efficient navigation between eBooks, but linear reading of them. You can't follow links between them, of course, because on current systems links must point either with the same eBook or out on to the internet---pointing to other eBooks on the same device is verboten. OpenURL can solve this by catching those URLs and making them point to local copies of works (and thus available for free even when the internet is unavailable) where possible while still retaining their