The Book of Protection (Paritta) translated and explained by Piyadassi Thera (1999; 200k/67pp.)
An ancient anthology of paritta (protection) suttas translated into English: discourses of the Buddha commonly recited for protection against fear, illness, and misfortune. An introductory essay explains the value of paritta recitation for physical and mental health.
The Seven Factors of Enlightenment [Browse]   [PDF file] [PDF file]
by Piyadassi Thera (Buddhist Publication Society Wheel Publication No. 1; 1960; 62k/18pp.)
This essay offers an overview of the seven factors of enlightenment (satta bojjhanga), one of the Buddha's own lists of his most important teachings.
"The Fact of Impermanence," by Piyadassi Thera, in Three Basic Facts of Existence, The: Volume 1, Impermanence (Buddhist Publication Society "Wheel" No. 186/187; 1981; 152k/43pp.)