NUL provides some valuable resources for assessing library collections. I especially like their Data & Methods Bank – a LibGuide with sets of data just waiting to be used. Methods listed include mapping their collections to the university’s curriculum, fund allocation strategies, and qualitative. Links are provided to data sources specific to NUL, like the NU Data Book and university rankings, as well as general library & publishing statistics. There is even a section on altmetrics.
This is a good resource to use as a starting point when considering any kind of evaluation of your collection.