How does a knight increase a skill that starts at zero? Or can he never do so?

Several skills start at zero and The Book of Knights states that non combat skills cannot be augmented. Does this mean, for example, that a knight can never read?

Also, and this is not related to skills at zero, the Book of Knights states that no male character can start with the Chirurgery skill (and KAP5 lists it as non knightly), however, in the detailed skill lists, Roman knights have Chirurgery starting at 5. I assume this is an exception.

The limit on starting skills at 0 applies only during character design. once in play you can use winter phase training to train up any skill at all.

Yes, Roman knights are an exception for Chirurgery.