Lethbridge's next book was "ESP – Beyond Time and Distance", published in 1965. It dealt with the theme of extra-sensory perception and articulated Lethbridge's argument that rays of energy were transmitted from every object, and that they could be detected using pendulum dowsing. In 1966 he published "A Step in the Dark", which repeated many of his theories regarding pendulum dowsing present in earlier works. Early that year, Lethbridge first began struggling against heart disease, an affliction that had resulted from his obesity. In both his books and private letters from this period he regularly ranted against modern life and society, while in other correspondences with individuals in the United States he championed the authenticity of the Kensington Runestone and Westford Knight.