Your sentences about the new books are OK.
But you cannot say
公園で人々と何人か話していた。
Because you cannot "talk some" with people.
This sentence sounds like
In the park we were talking with people how many people (are there, or did something, etc.)
Unlike English, you do not have to make 人 into plural.
何人かの人 is OK. (何人かの人々 is grammatically correct, but do not sound very natural.)
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