David Brin 
Sundiver [EPUB ebook] 

Circling the Sun, under the caverns of Mercury, Expedition Sundiver prepares for the most momentous voyage in our history. A journey into the boiling inferno of the sun, to seek our destiny in the cosmic order of life.For in a universe in which no species can reach sentience without being ‘uplifted’ by a patron race, it seems that only mankind has reached for the stars unaided. And now, the greatest mystery of all may be explained…Sundiver is the first book in David Brin’s magnificent Uplift series.