The Key to Liberation, in its original Thai form (Gunjaer Bhavana), was the very first of Ajahn Chah's teachings to appear in print, sometime in the 1960s. This new translation into English was done in 2002.
The following Dhamma talk was informally given to a visiting scholar monk who had come to pay respects to Venerable Ajahn Chah.
The Key to Liberation //www.dhammatalks.net/Books2/Ajahn_Chah_The_Key_to_Liberation.htm
The Collected Teachings of Ajahn Chah //www.buddhanet.net/pdf_file/teachings_chah.pdf