Three powerful Sci-Fi's:

Enchantress from the Stars; Singularity; and Anna to the Infinite Power.

I was completely engrossed and awed by Enchantress, which was written by Sylvia Engdahl. It's complexity and insight truly shows the dexterity of an able writer. Told from three distinctive perspectives, each from a different "level" of civilization -- somewhat equivelant to early Earth European civilization, Star Treks time, and a far more advanced and superior one. Three characters each sees and explains the same incidents from her or his own frame of references. Simply marvelous.

In Singularity, Harry, the thoughtful and more introverted of the twins, uses the time difference of a singularity field to age himself one full year older than his brother, Barry. The passages describing his solitary year are fascinating. William Sleator is such a good storyteller with in depth knowledge of the scientific theories he bases his stories on.

Mildred Ames' Anna to the Infinite Power is a thoughtful, challenging, and chilling account of the possible impacts of Human Cloning to the individuals that are clones. Anna grows from an amoral scientific genius who couldn't care less about other people's feelings to a frightened, and then compassionate human being. Even though that the 90's described in the book (which was published in 1981) is quite different and far more advanced, technologically, than the 90's we are experiencing now, the emotional aspects of the characters are more than convincing. It keeps the suspense up till the very last page. Even when you think you know the Ending of the book, you are confronted with one more heart wrenching blow. The questioning of what the Government does to its citizens and the parallel depiction of the scientists and the Nazi concentration camp could have caused some controversy and definitely can provoke serious discussions.