My 7-year-old son has been reading these short chapter books on super DC Comics super heroes from the library, and I thought I'd recommend to anyone looking for a doable chapter book for their young readers.
The books are all published by Stone Arch books and are five chapters long (between 45-50pp). It's a good length and the vocabulary isn't too daunting. My son is able to read them, get excitement, good pacing and decent story.
The DC comics series focuses on Batman, Superman and Wonder Woman. Here's a link to Stone Arch's info on the books: http://www.capstonepub.com/aspx/pcategory.aspx?EntityGUID=5ae6c501-67e8-4f89-8534-af34802cf11c.
Remember, you don't have to buy them, as they're available at the PG County library. But, definitely worth checking out for your young reader.