A bench-warmer who became a star; Quarterback Walk-On, by Thomas J. Dygard. New York: Morrow Junior Books. 221 pp. $8.50. (Ages 12 and up.)

This novel may never be considered the acme of great sports literature. But the story of a fourth-string college football player who suddenly finds himself quarterbacking a national powerhouse team is rather fun reading in the ''books for boys'' category.

Denny Westbrook, ''a shaky passer, and uncertain ball handler,'' was as shocked as the rest of the team when he received word that, due to the suspension of one player and injuries to two others, he would be starting in the next week's important game. He knew, and the reaction from many of his fellow players confirmed, that the only way he could ever even hope to lead the team to victory would be to come up with a new strategy - so he did.

There's football action, suspense, and fairly realistic interplay among the characters in this book. In addition, young sports enthusiasts who don't read much else just might get hooked on reading. Even nonenthusiasts might stick with this book, thanks to an appealing hero. As his girlfriend tells him encouragingly just before the big game, ''You are every undersized fourth-stringer in the world. You are one of them - the ones who never got to be the star of the game - and they are glorying in every moment of your triumph.''

You can't help but pull for a guy like that.