Ven. Buddhadasa Bhikkhu (1906-1993) has been an influential religious leader in Thailand. He played a remarkable role in Siam-Thailand transformation of 1932, followingly inspired a society-concerning generation of 1960s. His teaching has been focused on voidness. Through examining voidness, he conveyed the relationship between Dharma and ego, moreover, related the voidness to ordinary practice at times of contact and death. For me, I enjoy reading his teaching a lot, after few pages I'd close my eyes and ponder over, slow and beneficial for mind.
Heartwood of the Bodhi Tree is recommended for understanding the voidness, dhamma in Bhuddist teaching. (Please be kindly noted that the link provided is not identical to the book, but a collection of Buddhadasa's talks. However the essence of his teaching should be identical.)
For friends who have chanced to enter temple, immersing in painting and transcriptioin, please enjoy the online pdf file Teaching Dhamma With Pictures. It is really fun and meaningful.