Which is correct,"who/whom do you want to be?" In one book I found that "to be" follows who and not whom, but the sentence should have an objective pronoun whom if we go by the normal rules, as it is the object here.
"Normal rules" in this case depend on level of formality and fidelity to the origins of English grammar. Traditionally, prescriptively, the copula "to be" isn't a verb that takes an object. The answer to "Who is that?" asked in the darkness on hearing someone approach might be "It is I, the Scarlet Pimpernel!" "Who do you want to be?" is consistent with that.
People don't normally follow that rule for the copula in English these days, at least not in all but the most formal, hypercorrect speech and writing, and treat "to be" as though it takes an object. If you, accordingly, would say "It's me", then "Whom do you want to be?" would be consistent with that ...
... BUT: "Whom" itself has largely become a historical curiosity. Few people would ask "Whom did you buy that gift for?" One is much more likely to hear "Who did you buy that gift for?" (And let's, please, not even get into an argument over ending sentences in prepositions. That was a manufactured rule.) Few could articulate what "whom" even means. In that context, "Whom do you want to be?" sits on the fence between upholding traditional, grammatical purity and reflecting the way people normally speak today, unable to decide which side it wants to be on.
The first word in the question "Who do you want to be?" is not an object. It is a predicative complement. Complements of the copular verb be are not considered to be objects.
Because the subject of be in this sentence corresponds to the "nominative" pronoun you, tradition prescribes putting the predicative complement in the "nominative case" as well: you should use who and not whom, as the book you read seems to have mentioned.
When be has an accusative pronoun as its subject, tradition prescribes putting the predicative complement in the accusative: "Whom do you want him to be?" (In other situations, it gets more complicated.)
I put "nominative case" in quotation marks because it's highly debatable whether modern English actually has any kind of case system at all—and if it does, whether "nominative" is an appropriate label for the case used in this context. But that's the traditional name used to refer to forms like I they who as opposed to me them whom.