I saw many types of 3-phase power transformers such as: delta/Wye or Wye/Wye or Wye/delta also a transformer can include many primary and secondary coils for each phase not just one primary coil and one secondary coil.
I know how a transformer works but I wonder why there is too many types of transformers as I explained above !

Would you suggest me a free book or website that explains in details the types of power transformers, please ? I googled on the internet but I found poor results.

 

Search for 3 Phase transformer theory?

Google really is your friend!

 

It is not just about transformers, it is also about rectifying systems. I have an old Motorola rectifier manual that has a, "Poly Phase" section in it. This is my "go to" book for questions on Delta/Wye transformers and rectifiers.

 

Look for 1990's or older book put out by Emerson, called "drives and servos yearbook "

https://books.google.com/books/about/Drives_and_Servos.html?id=luOvAAAACAAJ

There are about a billion (gross exaggeration) types of transformers and that book covers just about all of them.