This is the 442nd most frequent Hebrew word.
ידוע
Known.
Here, 'ידוע' is again an adjective, qualifying the noun 'ספר' (book) by indicating it is recognized or renowned.
זהו ספר ידוע בכל העולם.
This is a famous book worldwide.
In this usage, 'הידוע' is a noun meaning 'the known', referring to something explicitly established or recognized.
הידוע הוא שמזג האוויר יהיה מעונן מחר.
The known fact is that the weather will be cloudy tomorrow.
In this sentence, 'ידוע' (yadua) functions as an adjective meaning 'known', describing the subject (he) as being recognized for his specific talent.