Picture StoryBooks are what most people think of as traditional picture books. Of late, book publishers are buying manuscripts with fewer rather than more words. Stories are more complicated, and are usually read to children. Pictures and words have equal importance in these books. Easy Readers are intended to be read by kids, not to kids.
Libraries often have books that reproduce appropriate vocabulary lists for school age children.
These are in great demand right now. Written for older children to read by themselves, but for children with low reading levels. They tend to be about sports figures, movie stars, popular culture, etc. They may have as few as 60 pages, or as many as 200. Middle grade novels are used extensively throughout middle school for book reports and in interdisciplinary lessons.