Hi, I bought and read the book Timelapse shooting..... and think it´s great. Good job!
Anyway, there is something that confuses me that is the interval shooting. page 37 and further you talk about the 180 degree shutter technique that I understand for video and films, but when it goes to the photos of a timelapse sequence, I don´t understand the following:
If you decide to have a 3 second interval between shots, does it mean that you need to use a 1.5 second shutter speed? dont the pictures look motion blurred if you take a sequence of people walking or cars passing by? 
And if the interval is 10 seconds, you have to shoot at 5 seconds? It seems too much to me
I have done some preliminary work on a sunset with clouds moving shutting at 3 second interval and 1/40 second exposure and the result is fine for me, why in this case shoot at 1,5 seconds?
in any case, the 180 degrees rule must be used always to get the best results?
is there any tutorial of videos exampling the differences?
your advice will be very well appreciated

The 180 degree rule is not a strict rule.
But it normally helps to get smoother footage. So yes it's meant to be 1.5 sec. exposure for a 3 sec. interval. And the purpose is to introduce motion blur. Try it out by your own: the results will look much better!
Sometime of course you might decide to use shorter shutter speeds (or even longer) depending on the subjects.
You will get experience with this, if you try.

Hello,

I also investigated a lot of time to play with different exposure times on same interval.

A video, the same scene made with 2 cams to the same time and different exposure times, would be really helpful to understand the reason of the 180 degree rule.

My impression of my entire tests was:
Especially for waves in the ocean or moving water (waterfall) I try to go as much as possible close to the 180 degree rule.
For fast moving clouds I don’t like the motion blur effect. So I’m using shorter interval times.
For walking peoples and cars, it’s depends on the scene. But I also think, the 180 degree rule is too much exposure time.

Thank you, one more question: for a starry sky at night, what interval would you choose and what shutter speed you recommend?

The shutter speed should be as long as it can get without making the stars look like lines - this depends from the focal length.
So with a ultra wide angle (14mm on full frame) you can get up to 16 seconds exposure time.
The interval should then be at least 18 seconds to give the camera some time to settle and write the image.

and what´s your opinion about using variable ND filters? are they as good as individual ones?

Variable ND filters are a no go for time lapse!
They are built from two polarization filters that are twisted.
If you make them dark enough, you will get nasty and uneven shading effects.
I do not recommend using them for time lapse at all!