Mindfulness in Plain English is a classic and one of the most popular explanatory texts for Insight meditation and Vipassana. Originally published in 1990 by Ven. Henepola Gunaratana, the book has taken various shapes and sizes. The work has been placed into public domain free-of-cost.
This compilation for iBooks is in humble reverence to Ven. Henepola Gunaratana, and to my own Vipassana teachers at dhamma.org.