For a toddler or a preschooler, seek out books that have brightly colored illustrations of everyday objects and friendly-looking animals. Look for textures and durable binding. Don't concern yourself so much with plot development, theme or character, although books on subjects such as counting and the alphabet are good for this age group as well.
For kindergarteners, look for very simple stories. Seek out books featuring rhymes and other text that sounds appealing. Since kindergarteners are just beginning to understand and enjoy plots. Look for simple plots with happy or educational endings, such as Aesop's fables or non-scary fairy tales. Look for colorful imagery and stories involving familiar objects, such as household items and animals.
For children beginning to read on their own, look for strong storylines told with simple words. Choose books with fewer illustrations than those for younger children. Look for simple humor, such as that found in series such as "Captain Underpants," "Flat Stanley" and "Sideways Stories from Wayside School."
For kids who are strong readers on their own, look for action-packed fantasy and science fiction, as well as family themes. For kids in this category who are going through a major life change--such as a divorce or the death of a family member--look for books dealing with characters going through a similar life change.