What is the collective noun for library?

There isn't a commonly used collective noun specifically for a group of libraries.

Here are some options depending on the context:

General:

* A collection of libraries

* A system of libraries

* A network of libraries

More specific:

* A chain of libraries (if they're owned by the same organization)

* A consortium of libraries (if they collaborate)

You could also use a more general collective noun for buildings, like:

* A cluster of libraries

* A row of libraries

Ultimately, the best collective noun to use depends on the specific situation and what you want to emphasize.