Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
The human brain
Ink and paper
Words on a page
A match being lit
A light bulb coming on
A journey that never ends
SEE ALSO: A Quest For Knowledge (we loved knowledge so much we did it twice!)
These are just a few examples of things one might associate with knowledge. Some are more powerful than others. A library is a strong symbol, and likely will not require reinforcement. However, a diploma could stand for a number of different things and not foreshadow knowledge on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.