Her female characters are amazing. They are strong, capable and not reliant on a boy. Amy from 'This time of Darkness' is the best 11 year old you will ever read about, she drives the narrative of that book and though there is a male character, Amy is the one who makes things happen.
In addition to her fascinating characters, HM Hoover has that wonderful gift of writing thought provoking fiction, without making it dull. Though her books are slim, easy reads (as most child/YA fiction was in the 80's/90's) she does not ignore big issues, even if she had to allude to them. Children of Morrow does not shy from physical abuse. Orvis deals with parental abandonment and Return to Earth shows us a corporate future.
It is an enduring source of annoyance to me that these absolutely wonderful books are not available anymore.
I would recommend anyone to pick up her books and read them. My personal recommendations are:
This Time of Darkness: Amy and Axel must escape the oppressive, underground city in which they live and find the 'outside'.
Children of Morrow: Following an ecological disaster, two different societies survive. Morrow is technological with access to telepathic abilities, the Base falls into misogyny and religion. When two children are born on the Base with telepathy, they come into contact with Morrow.
Return to Earth: Galen Innes, the retiring Governor of Marsat returns to his home and meets Samara, the sixteen year old daughter of his boss. When Samara's mother is killed, Galen helps Samara take up her responsibilities. (it's so much better than I make it sound - I don't want to spoil the book)