I'm seeking the name of the book, events and concepts I remember as follows (in no particular order).

• I read the book circa 1999 but it could have been written anytime.
• Setting: Mediaeval style with magic.
• Not everyone can use magic to the same degree, some almost have no magic, those that do are either magic users, or catalysts which cannot directly use magic but enable magic users to fully use it (act as a battery).
• There is a man and a woman that want to have a kid the traditional way even though it is illegal.
• At some point the main character is "sneaking" through fields where it is noted that minor magic users are tending the fields of food with their limited power (presumably under the direction of some organization?)