Basic questions regarding Push Pull stage
According to the books this is a voltage follower stage i.e Vo will be
  equal to Vi


No, Vo will never attain the level of Vi because of loading effects on the output and the need to exceed the gate threshold voltages to be able to turn on (to any reasonable extent) either transistor.

So, if Vi is 5 volts p-p and the power rail is 5 volts, then I would expect to see maybe 4 to 4.5 volts p-p coming out in no load conditions. Why not 5 volts p-p you might ask - the transistor that is "off" will still draw several nano amps and have capacitance from drain the source and this means real current has to pass through the "on" transistor to deal with the loading effect of the "off" transistor. This means the source cannot truly follow the gate without some volt drop.

To better understand this circuit get rid of the P channel MOSFET and replace it with a resistor to form a source follower amplifier - gain is somewhat less than 1 and this is the same basic problem that is faced in the push-pull version (but it is slightly improved in no-load scenarios).