My book says above that there are three essential nodes in the circuit above. I am seeing four essential nodes in total which are marked by those four black dots. Since each of those black dots connect three or more circuit elements? Can someone explain to me how there are 3 essential nodes instead of 4 essential nodes?
Black dots do NOT indicate nodes. They are visual markers.
A node is the complete interconnection between circuit elements. There are four nodes in your circuit.
An essential node a node where three or more elements meet. There are three essential nodes in your circuit. The bottom-most node in your circuit is one of three essential nodes.