Sundara Kanda (Sundara Kāṇḍa), literally “beautiful episode/book”, is the fifth book in the Hindu epic the Ramayana. It depicts the adventures of Hanuman. The original Sundara Kanda is in Sanskrit and was composed by Valmiki, who was the first to scripturally record the Ramayana. Sundara Kanda is the only chapter of the Ramayana in which the hero is not Rama, but rather Hanuman. Hanuman’s selflessness, strength, and devotion to Rama are emphasized in the text. Hanuman was fondly called Sundar by his mother Anjani and Sage Valmiki chose this name over others as this kand deals mainly with Hanuman’s journey to Lanka.

Source: Wikipedia