Elephant & Piggie is a series of children's picture books by Mo Williams. You may recognize this name; he made quite a splash the the Pigeon books a few years ago (starting with Don't Let the Pigeon Drive the Bus!, 2003), and has written dozens of other picture book, generally with simplistic, humorous themes.
The Elephant & Piggie series is more of the same, but better. Piggie (a pig) is a hyperactive young girl with a bubbly personality. Gerald (the elephant) is a rather more dour, but kindhearted, young boy. The standard story arc involves Piggie being very excited about something, and Gerald following along with varying degrees of reluctance or exuberance. A problem is encountered, they both overreact, they agree to work on a solution, and a happy ending is shared by all.
This sounds a bit boring... but it isn't. Piggie and Gerald are very expressive, unintentionally comedic, and are just generally a wonderful satire of the life of a five-year-old. While they are perhaps not quite so wonderful that I recommend that you all go out and read the series, I do recommend that if you have a young child, or find yourself with time to spare in the children's library, you check them out. They are good fun for all ages, and will probably do your kid good.
Speaking of which... The books are written with simple language, large print, and lots of illustrations. They are not bad as beginning reader books, although that is not their primary purpose. They have messages of "be nice to your friends" and "don't just give up, at least try" that are fairly milquetoast and are clear without being preachy. They are highly conductive to theatrical readings. And most importantly, they are books that you will enjoy as much as the child. If your child is of an age to enjoy having picture books read to them, Elephant & Piggie is likely to be a good choice.
There are 25 books planned before the series is officially complete (only one left to go!). The stories are independent, so the books can be read in any order, and there is no need to plan on reading them all. By publication date, the books are: