The possibilities are vast and this is a small sampling:
To find books and information on library and archival holdings around the world, try:
The Penn Libraries provide multiple ways to connect with books and journal articles that aren't immediately available to you in print or electronically, including:
BorrowDirect -- books from our Ivy league partners and a few others including Stanford and University of Chicago
EZ Borrow -- books from our Pennsylvania Partners and a few others including New York University
Scan and Deliver / Interlibrary loan -- for journal articles, chapters in books, and materials not available through our partnerships.
These compendiums of knowledge on specific areas and topics are excellent ways to gain familiarity on a topic. Volumes are edited by specialists, and each chapter will lead you to additional secondary readings and primary sources.