There are some good lecture videos with Dijkstra online that give a better introduction to his way of thinking than anything he wrote:
"Reasoning about programs": https://www.youtube.com/watch?v=GX3URhx6i2E
"The power of counting arguments": https://www.youtube.com/watch?v=0kXjl2e6qD0
"Structured programming": https://www.youtube.com/watch?v=72RA6Dc7rMQ
"NU lecture": https://www.youtube.com/watch?v=qNCAFcAbSTg