Would it? If all books were available in DRM-free formats, you could replace an entire building with a single computer and some offsite backups. The energy use for lighting, air conditioning, elevators, and everything else that goes into running a library is non-negligible. Not to mention the embodied energy of all the materials for building and maintaining a large structure for 100 years.

That's not to say I'd support replacing libraries with an ebook lending system; they serve important functions beyond book lending, and I like paper books anyway. But I wouldn't claim that they're low energy.