Sentence Correction

dear experts,
would you please clarify that pronoun in modifier is always incorrect?

for example:
The book that I read it yesterday was exciting-- pronoun "it" is redundant, because that stands for "the book", it stands for "the" book as well, -- incorrect

is the following sentence correct?
IMO , it is correct, but I am not sure, so I desire your confirmation
Nobody knows exactly how many languages there are in the world, but those who try to count them have found about five thousand.
in this sentence, second clause's subject is those, modifier is who try to count them, and there is no any redundant in the second clause, so IMO, pronoun them is correct.
please confirm.

so silly question,
thanks for your patient.

have a nice day
>_~

Hi Zoe.

The idea that there can never be a pronoun in a modifier does not make sense. Phrases and clauses that have subjects and objects can be modifiers, and pronouns can be used in the phrases and as the subjects and objects of the clauses.

In your example, the use of them is correct.