The Dan Frontier series (older, OOP and worth the search. Graded series of readers)
Cowboy Sam series (ditto)
Jim Forest series (ditto again)
Books by Clyde Robert Bulla – The Sword in the Tree, Viking Adventure and many more. Some have been reprinted but look for the older ones with larger print. Excellent, excellent stories, vocabulary, and story-telling overall. These books have single-handedly turned reluctant reader boys into voracious readers.
The Eddie series by Carolyn Haywood
Garrard Discovery biographies by various authors – an older series of bios superbly written for beginning readers. My favorite bio series for the youngers. Here’s a list of them
http://www.valerieslivinglibrary.com/garrard.htm