The Jack series, a set of bright, handsome board books, let the friendly dog Jack (is he a fat terrier?) teach toddlers to four-year-olds basics such as colors and shapes only they won't know they're learning because they'll love lifting the flaps on each of the six spreads for the surprise. The text is very short, simple, logical, and fun. In Jack, It's Bathtime (Kingfisher, $5.95, 0753451409), the little reader pulls down flaps to find Jack's yellow duck, his blue fish, his red sailboat, and his green towel. Then they all appear with Jack in the tub in a pop-up announcing it's bathtime for Jack! Lots of good thinking went into this series. There's plenty of space for little fingers to pull-down the flaps, and the similarty in format from one spread to the next doesn't call for extra time to search out the manipulative element. The bold black of the few words and in the lines around the illustrations make comprehension easy. Other titles in the four-book series cover bedtime, playtime, and the bonanza, Jack's birthday. Alice Cary reviews books from her home in Groton, Massachusetts.