Source: The Great Interpretation of Dreams (Ibn Sirin)
A library in a dream indicates knowledge, wisdom, and sitting with scholars. Entering it indicates seeking knowledge. Buying books from it indicates gaining beneficial knowledge.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
A library indicates a scholarly man or a righteous woman. Sitting in it indicates guidance and righteousness.
Source: Signs in the Science of Expressions (Ibn Shaheen)
A library is interpreted as goodness and benefit. Whoever sees himself owning one will gain vast knowledge.