Personally, I don't consider e-libraries as CoPs for several reasons.
First, they seem to me to be storehouses of explicit knowledge. Second, they are organized so individuals look through them for their own purposes, consulting librarians when they need help (but often only searching the databases themselves). Third, the knowledge domain of an e-library is too broad, for it is not a group of people focusing on a defined knowledge domain. Fourth, although there may be some community aspects to setting it up and
managing it, the users are mostly individual.
However, they may be another point of view. The librarians themselves could be considered a CoP with the users as boundary members. For example, the librarians seek to advance library science itself as a knowledge domain. Yet most of these boundary members/users themselves don't seek to become core members (expert librarians).
That's my opinon only. I'm curious about other viewpoints.