Several other notable university libraries didn't make our top 20 list but are worth mentioning:

~20 million volumesHarvard doesn’t just top the list; it occupies a league of its own. Established in 1638, it is the oldest library system in the United States. It isn't a single building but a massive network of over 70 libraries. The crown jewel is Widener Library , which houses miles of stacks—many of them underground—containing some of the rarest collections in the Western world. 2. Yale University Library (New Haven, USA)

The United States dominates the list of largest academic libraries, a legacy of aggressive acquisition policies during the 19th and 20th centuries and substantial endowments.