What do you advice me to do? read a book about unity or the manual?

Why not do both? But the online documentation is very good.

Most important is practical experience. Start with very simple projects. Not actual games. Make a ball roll around. Make some dominoes topple. Make a nice looking scene. Do some tutorials. By starting simple, you'll avoid getting buried in complexity.

What I have found is the books will get you to level 1, let's say. After that, when you are deep in your development and run across a tough situation the books turn out to be useless. A good first step though. The coding examples provided here are the deepest most complex you could get. Like getting a set of vectors from mashing two sets of vectors together. I have found if you need an answer or idea just google: Unity3d blah blah blah. And viola' Tons of links to very indepth useful coding examples or designs.

I would prefer the online ressources. Better searchable and very rich of all kind of info though sometimes a bit confusing in it's wealth. Use this, this and this to dig up stuff.