There is some information in Jensen's Ben Johnson bio not readily available on the web, mostly about his parents and Ben's early years in Oklahoma and stunt years prior to meeting John Ford. The errors I've found in it mostly have to do with incorrect movie plot descriptions or some other minor details. But Jensen didn't have an axe to grind with Ben, as far as I can tell.
The John Wayne bio sounds like another kettle of fish entirely, however. (I haven't read it.)