James Blish 
Star Dwellers [EPUB ebook] 
Heart Stars Book 1

The inhabitants of Terra nicknamed them "Angels". They were exquisitely beautiful, these shimmering, fiery creatures , highly intelligent and playful. Yet they were awesome, too, considering that the youngest were four million years old, and the oldest had probably participated in the First Cause, which had given birth to the whole universe. To young space cadet Jack Loftus fell the overwhelming responsibility of negotiating a treaty with them – a treaty which could mean the life or death of earth and mankind.