This year I came across these books from Scholastic. They are from the series called A True Book. They are available in a wide range of historical topics, places, and people. The readability is fairly easy (not sure of the level) and they are jam packed with text features. I used The Pony Express book when we were studying text features earlier this year. I only own these three right now, but my dream is to collect a class set on a variety of topics.
Here are some pictures from Amazon (sorry that I forgot to take pics of the pages in the books I have).
Like I said, every book is full of text features...captions, bold words, primary sources, maps, charts, etc. Additional resources (books and websites) are listed in the back along with a glossary. They are short paperback books which makes them perfect to use as a mentor text. It took about 3 days to read through the Pony Express and discuss the features. I used them to model other nonfiction reading strategies, like asking questions. I can see myself getting a lot of use out of a class set. If you can't get a class set then have a couple on hand for your classroom library and as mentor text for modeling the reading of nonfiction.
Here are some other books in this series: