A little while back we read through A Heart Strangely Warmed, a Sonlight scheduled Core 7/H history read aloud.John Wesley's life/ministry has always interested me so I was pretty keen to dive into that book. Hold the "keen" Chelle. We ended up manfully (or womanfully in my case) plodding our way through the entire book.
I knew I'd read a John Wesley biography for children some years earlier that I'd enjoyed much more than the book SL recommends so I went scouting through our church library, found it, and came home with Horseman of the King: The Story of John Wesley by Cyril Davey.
If you're looking for a copy of Horseman of the King I found bookdepository has a really reasonably priced paperback copy, with free postage. Christianbook . com sadly don't stock it at all - yet.
I've read through the Benge's biography on Wesley and came away with a meh taste in my mouth ( I usually like Benge's books): that book made John Wesley come across as a womaniser - a mind set promoted by his Wesley's disgruntled, and fiercely jealous wife who did everything she could to damage his ministry and influence. (Being married to a Wesley must surely have been a challenge without having a green eyed monster tossed in to wrestle with!)
Update 2015: We're about to go through a study at a deeper level of John Wesley and have chosen to use Christian Leaders of the 18th Century : John Wesley ~ J.C Ryle, and read through some of Wesley's own journal entries. Wesley is such an important figure in (our) church history, that I do wish some one would write a *more accurate* biography for our youth to read... hopefully before we could read it during June 2015 ☺.