I have found a few books written by Roger Crab on Google Books... regarding his diet of roots and herbs... I cannot see the content...
Some of his story has been reprinted in 1813 by E.F. Mengel... and later by James Caulfield and James Granger...
The account of this singular character is chiefly comprised in the title to his life, which is reprinted in "Morgan's Phoenix Brittannicus," and runs thus.
The English Hermit, or the Wonder of this Age; being a Relation of the Life of Roger Crab, living near Uxbridge, taken from his own mouth, shewing his strange, reserved, and unparalleled kind of Life, who counteth it a Sin against his Body and Soul, to eat any sort of Flesh, Fish, or living Creature, or to drink any Wine, Ale, or Beer. He can live with Three Farthings a Week. His constant Food is Roots and Herbs; as Cabbage, Turnips, Carrots, Dock-Leaves, and Grass; also Bread and Bran, without Butter or Cheese. His cloathing Sack-cloth. He left the Army, and kept a Shop at Chesham, and hath now left off that, and sold a considerable Estate to give to the Poor; shewing his Reasons from the Scripture: Mar. x. 21. Jer. xxxv.—Wherefore if Meat make my Brother to offend, I will eat no Flesh while the World standeth," &c. 1 Cor. viii. 18.
Dr. Cheyne, who was an advocate for the vegetable diet, and mentions the longevity of some of the ancient ascetics of the desert, who lived on that kind of food, probably never heard of this strange humourist or if he did has passed him over in silence as a madman, who seems to have destroyed himself by eating bran, grass, dockleaves, and such other trash as was comprehended within his pious plan of living for three farthings a week. If Crab had resided in France or Italy, he would indubitably have retired to the monastery of La Trappe.
He probably died in London, as the following memorial of him is preserved in the church-yard of St. Dunstan, Stepney:
"Here remains all that was mortal of Mr. Roger Crab, who entered into eternity the 11th day of Septemb. 1680, in the 60 year of his age.
Tread gently, reader, near the dust
Committed to this tomb-stone's trust"