Probably the closest thing would be the Konungs skuggsjá (King's Mirror) a Norwegian text of instruction from the mid-1200s for a king. It takes the form of a dialogue between a father and son on politics, morality, warfare, and the governing of a society.

The essence of the advice is: collect lots of weapons and use them frequently to figure out how they work. Train at least six days a week in heavy armour with heavy shields and weapons to build strength and stamina. Look up friends and recommended fighters to fight with as often as you can, and learn the guards and tactics they use that work better than yours. Keep an extra set of armour and weapons so that they have no excuse not to train. Good universal advice.

Here is a translation on the parts which directly address training for warfare, including details on mounted combat; http://www.deremilitari.org/RESOURCES/SOURCES/kingsmirror.htm.

Also a short article very worth reading on how this fits into the general picture of military history; http://www.mediumaevum.com/75years/specregal.html