Hello Farhana ''This is the book that I told you about'' -- this would be the more common way of writing it

The source of your confusion is the necessity of choosing a proper preposition to go with the verb "to tell". Do you "tell about" or "tell of"? Similar confusion accompanies the verb "to speak", and the prepositions that go with "tell" and "speak" vary, which makes things even worse. In the future, you can avoid these difficulties by instead using the verb "mention" which is made for the exact purpose you have in mind. It needs no preposition at all: "this is the book I mentioned (to you)", "he's the guy whom I mentioned". All your choices are grammatically correct except that "who" cannot be the direct object of the preposition "about". You need to say "about whom". I agree with the others that "about" sounds better than "of", but I'm not sure why that is. "Of" is not wrong.

‘Guy’ is pretty informal, so it doesn’t match well with ‘whom’ even though it is grammatically correct. What I would most often say are: This is the book I told you about. He’s the guy I told you about. (I would need a good reason to break up ‘He’s’- I.e. one of either ‘He’ or ‘is’ needing emphasis.)

In #2, you could use either "who" or "whom." If you want to faithfully follow the rules of grammar it should only be "whom", but almost no one uses "whom" anymore in real life. Also since the relative pronoun is replacing the object of the second clause, it's optional. You could also say it without either "who" or "whom." He is the guy I told you about.