There friends,
I believe that Stevenson book and the old Westhinghouse book are good elementary references for transient stability analysis. However, there are other reference that are more up-today that include modern static excitation system, automatic voltage regulator (AVR), modern and faster governors, power system stabilizer (PSS), modern power electronic devices and new modeling techniques vital for the transient stability analysis.
In addition to the other references mentioned above, I suggest to consider the following:
a- Power System Stability & Control (EPRI) by P. Kundur.
b-Power System Control & Stability (IEEE) by P. Anderson & A. Fouad
c- Understanding FACTS (Flexible AC Transmission Systems) by N. Hingorani & L. Gyugyi.