I'm trying to find an old book I read. Aliens invaded Earth and almost wiped them all out. Small groups banded back together and reverted to more primitive ways. One human went off and got captured and the alien taught him to read and write and eventually they got other humans and it continues until they are fought back.
It could be a lot of books. One possibility is Battlefield Earth by L. Ron Hubbard.
The novel follows the adventures of Johnny Goodboy Tyler.
The Earth has been invaded and conquered by the alien Psychlos.
The current head of mine security (named Terl) for the Psychlo operations on Earth captures Johnny and teaches him to read, write, and speak the Psychlo language.
Terl wants to use Johnny to head a group of humans to mine gold for him - personally and illegally. He has a plan to smuggle the gold home so that he can retire rich when he is transferred back to Psychlo.
Johnny manages to free the Earth, and incidentally destroy the planet Psychlo. Due to the way transport and communications work, this also wrecked all the Psychlo colonies and mining operations on all conquered planets - Johnny freed not just the Earth, but multiple planets in many galaxies and universes.
It is a fun tall tale to read, just don't try to take it seriously.
It is also an enormous tome. The paperback copy my father had was over 1400 pages. It was nearly four inches thick.
This is Battlefield Earth by L. Ron Hubbard.
After one thousand years, humanity is an endangered species numbering fewer than 35,000 and reduced to a few tribes in isolated parts of the world while the Psychlos strip the planet of its mineral wealth. Jonnie Goodboy Tyler, a young man in one such tribe, lives in the shadow of the Rocky Mountains. Depressed by the recent death of his father and both the lethargy and sickness of most of the surviving adults in his tribe, later determined to be caused by radiation-leakage from decaying nuclear land-mines, he leaves his village to explore the lowlands and to disprove the superstitions long held by his people of monsters in those areas. He is soon captured in the ruins of Denver by Terl, the Psychlo chief of planetary security.
The book was made into an award-winning film.