Even though Keen's book is not the most recent book on the topic, he still has some very good things to say. This edition republishes all his original material with a new introduction, explaining how some of his ideas have changed due to recent scholarship. He has a chapter on outlaw ballads as an expression of peasant discontent, but has stated that he may have overstretched his conclusions. However, his book is still good to read in order to gain a broad understanding of English outlaws, he focuses on Robin Hood, but mentioned several other outlaw poems as well. For me personally this was the first time I read an overview of the life of Hereward and the tales of Gamelyn. I've heard of William Wallace, but didn't really know anything about him. Keen also has a chapter on Eustace the Monk, one of the outlaws who can be tracked down and pinpointed to exact dates.
His chapters on Robin Hood outline the fictional character and overview a few of the ballads. He notes how tough it is to try and find a historical Robin, and states that it may never be done. A few candidates have been put forward but they are all on shaky foundations and speculations. We know the rough century/century and a half in which the stories first began, so the original figure or figures who inspired them must have lived in the same period. Overall a good book if a little outdated by more recent scholarship.