Lord Hanuman was a devotee of Lord Rama and Mata Sita. He helped Lord Rama in the battle with Ravana. Lord Rama and Mata Sita loved Hanuman like their son. Hanuman was awarded immortality help and save devotees in Kaliyuga. We can find the presence of Lord Hanuman in various ancients texts which happened after Ramayana.
The hidden reason behind hanuman being immortal is that he is known to have knowledge of all the things in the universe and he is assumed to be the next Brahma.
Secret scroll of Gurutva: This book focuses on the secrets of gravitation and actual instructions on how to make the ancient Vedic Vimana, (like Vaimanika Shastra on aerospace technology).- Hanuman
Secret scroll of Sukshma Jeeva Shastra: This book focuses on microbiology and biotechnology.- Hanuman