I've mentioned Andre Norton on a number of other threads. She wrote really interesting, fast-paced page-turners which appeal to young {and older
} adults. Her books were quite popular in several school libraries I know about.
Generally she favoured fantasy and science fiction themes--both of which appeal to young adults. Norton's characters--both male and female have considerably more psychological depth than is common in these genres. She developed her own fascinating literary style which evokes the dark, medieval sort of fantasy landscape which is frequently the dominant type of fictional universe she creates.
Her novels are available in e-book format and a good selection can be downloaded for free from Project Gutenberg, Many Books, or Feedbooks in various mobile formats. Amazon includes her in several anthologies of science fiction, pulp literature and fantasy.