James Blish 
Mission to the Heart Stars [EPUB ebook] 
Heart Stars Book 2

Past worlds almost too strange to describe, three men hurtle into the dark uncharted mystery of space. Their task: to make contact with the Hegemony of Malis, the unknown force that has been watching the Solar System since before the beginning of recorded history. On their success or failure depends the future of the entire Earth. Join them as they accelerate on Haertel drive towards the stunning core of the Hegemony itself in James Blish’s masterpiece of the time to come, Mission to the Heart Stars.