Some of the books mentioned at Sunday Sangha:

Mindfulness In Plain English by Henepola Gunaratana

The Middle Length Discourses of the Buddha (PDF version here)

There are so many interesting and foundational teachings in this collection.  The core instructions on the practice of mindfulness are contained in discourses #10 and #118

Breath by Breath by Larry Rosenberg

Mindfulness with Breathing by Ajahn Buddhadasa

Heartwood of the Bodhi Tree by Ajahn Buddhadasa

Straight from the Heart by Ajahn Maha Boowa

Practical Insight Meditation by Mahasi Sayadaw

The Heart of Buddhist Meditation by Nyanaponika Thera

Living in the Light of Death by Larry Rosenberg

Full Catastrophe Living by Jon Kabat-Zinn