- We can know about Lord Hanuman from Ramayana but it states only a small portion of his life.
- Is there a scripture that tells about Lord Hanuman's story completely ?
Valmiki Ramayana has the life of Hanuman which happened in Treta Yuga only. But the life of Hanuman is indeed wide in scope. He has been immortal since ages. He is alive even today.
Why didn't Sage Valmiki focus on Hanuman like he did on Sita and Rama?
Ramayana doesn't mean the story of Rama but it is the life/history of Sita.
Balakanda, chapter 4 clearly states that.
काव्यम् रामायणम् कृत्स्नम् सीतायाः चरितम् महत् |
पौलस्त्य वधम् इति एवम् चकार चरित व्रतः || १-४-७
Meaning: That sage with observed sacred vows has rendered the entire epic in the name of 'Ramayana', 'Sublime Legend of Seetha' and 'elimination of Ravana'. [1-4-7]
So, Ramayana only discusses about Sita and Rama who is incomplete without Sita.
Then which scripture discusses the complete life of Hanuman?
The answer is Parāśara saṃhita. The seer of this scripture is Parāśara( Father of Krishna Dwaipayana vyasa). In this scripture, life of Hanuman before Ramayana and the life of Hanuman which happened in Dwapara yuga are also present.
But some aspects in this scripture are not accepted by some people. For example this book says Hanuman is married to Suvarchala, daughter of Sun God. On the contrary, our puranas, Mahabharata say Suvarchala is wife of Sun-God. See this question for details.
Parāśara saṃhita is also translated into Telugu and English languages.