From Merriam-Webster:

Learn in the sense of “teach” dates from the 13th century and was standard until at least the early 19th.

made them drunk with true Hollands—and then learned them the art of making bargains — Washington Irving

But by Mark Twain’s time it was receding to a speech form associated chiefly with the less educated.

never done nothing for three months but set in his back yard and learn that frog to jump — Mark Twain

The present-day status of learn has not risen. This use persists in speech, but in writing it appears mainly in the representation of such speech or its deliberate imitation for effect.

There are 64 books in the Horus Heresy.