Once they feast their eyes on the exterior of this mouth-watering volume, youngsters will be impatient to dig in. The front and back covers are designed--quite convincingly--to resemble pieces of bread, and the spine reveals the neatly layered ingredients of the special snack Sam prepares for his sister. The most delectable morsels, however, are found within--each spread depicts one of the foods the obliging brother adds to his culinary creation. But the realistic renditions of sliced tomato, cucumber, cheese, pastrami, onion and hard-boiled egg fold out to disclose some of Sam's secret--and less appetizing--additions, including a fly, worm, slug, caterpillar and some ants. Youngsters will be too engrossed in Sam's creative sandwich-making to notice that Pelham's singsong rhyme occasionally falters. This cleverly designed book accomplishes what it sets out to do: it serves up a tasty little snack rather than a full-course meal. Ages 4-9. (Sept.)