This is the 2768th most frequent German word.
manchem
"Manchem" means "to some" or "to certain" in English.
In this sentence, 'manchem' is the dative singular form of the indefinite pronoun 'manche,' referring to 'some books'.
In manchem Buch findet man interessante Geschichten.
In some books, one finds interesting stories.
'Manchem' here is dative singular, referring to 'some individuals', and is implicitly referring to a subset.
Manchem Menschen fehlt die Geduld.
Some individuals lack patience.
In this usage, 'manchem' indicates the dative singular, referring to 'some works of art'.