Hello TTP-Comp & welcome to MobileRead. You'll find lots of our members enjoy reading Science Fiction. I'll assume that you know about the free SF/F books that Baen gives away. And, Random House is now giving away 2 free books each month at its
Suduvu Free Library. The Suduvu site only has the PDF files, but both Amazon and Sony match the Suduvu free offer. (I haven't checked to see if Fictionwise or BooksOnBoard do the same.)