Raja - Please be more specific so we can help you out. Which introduction? I find the circuit in several places, Figs 8.57, 7.54, and 6.38 but nothing that says it is negative fb. Also, when I do the input impedance on that circuit it is negative.

I see several examples of problems (such as finding input impedance) with positive feedback circuits in his book. Generally you have to add resistance to keep these circuits from latching up or otherwise hitting a rail.

I'm glad you asked however as it reminded me that I need to read this book.