Without seeing the context, I think it's important to explain the different types of camera settings.
Before we had automatic exposure, the shutter speed and aperture had to be set (along with the ISO rating of the film). You were the light meter.
M - Manual mode.
You can set the aperture, shutter speed and ISO to your liking.
A - Aperture priority.
Digital cameras have a light meter and will automatically guess the shutter speed according to the aperture you set it to. This is all according to the cameras specs and will differ across all models/brands but in general, you'll get similar exposures.
Example - If you set f5.6, it will automatically set the shutter speed to 1/125 (depending on the lighting conditions). If you then set it 1 stop down to f2.8, it will set the shutter speed to 1/250.
While, at the same time if you set it to f5.6 and change lighting conditions the shutter speed will change automatically for an even exposure while the aperture will stay as a constant, hence Aperture priority.
S - Shutter priority.
Same concept, except you set the shutter speed.
While going through book, i came across few sections where he first choose "shutter speed" in place of the "aperture".
Is there any thumb of rule for these decisions, when to go with aperture or when to go with shutter speed or it just a decision based on experience.
For me, it was based on experience. For night shots and long exposures, setting the shutter speed to 1 second or up to 60 seconds and this is where you would set the shutter speed first.
Aperture changes your depth of field. If that's what you're after, a low f-stop will give you narrower depth of field.
So, really, is you're shooting in "A" or "S", you don't need to worry about getting the right exposure. Unless you want to use the EV +/-.