Karna, the son of sun was a benevolent person. Similarly, a child who lends his book happily to the other child who does not have a book is like Karna.
Karna had Kavacha and Kunal since birth, therefore he was invincible. Being benevolent by nature, he never disappoints anyone. Lord Indra wanted his son Arjuna to be victorious in the battle of Mahabharata. So he went to Karna disguised as a brahmin and asked for alms. Karna offered him cows, horses, elephants, gold, the whole earth and even his life. But Indra rejected all these things and asked for his Kavacha and Kundal. Shalayaraj tried to prevent Karna from doing so but Karna gave his Kavacha and Kundal to Lord Indra happily, as it is said, 'देयं तथापि कवच सह कुण्डलाभ्याम्'. Thus, this statement indicates the importance of donation of books.