I don't know if any of his books are available in e-editions, but I think Hal Clement was one of the best "hard SF" authors that I've read.
I found a list of authors who are often associated with hard SF at
http://www.hardsf.org/WritHd01.htm and
http://www.hardsf.org/WritHd02.htm Many of the authors that others have recommended are on the lists.