MacMillan McGraw Hill Publishers have quite a few simple biographies of famous figures in U.S., California, and World History.
The same site also has similar biographies of people who fit into other categories.
Each biography also provides links to additional accessible resources on the people described.
These biographies are good just as stand-alone reading or (and this is what I particularly like about them) being adapted as clozes (fill-in-the-gap) exercises.
I’m adding them to The Best Resources For Researching & Writing Biographies.