If i am understanding your question correctly, and my own understanding is correct, think of it as two seperate sentances: "Of which is..." the best book. - Of which is singling out the book amungst other books (plural) and saying it is the best. "That is..." the best book. - That is the best book is merely saying it is the best book, it could also be the only book!

I have surrfed many websites ,but I couldnt undersatand ,I mean when to choose one of them in a sentence like this one: Me: Lately I made some decisions, (which) might change my life The correction: Lately I made some decisions, (that) might change my life.

Why don't you create some examples, or google grammar for which or that?