An affix comes after a word in the Instrumental -3rd case in construction in the sense of 'made', the thing made being 'a book'.,
Thus वररुचिना कृताः = वाररुचाः श्लोकाः 'the verses composed by Vararuchi'. So also हैकुपादो ग्रन्थः, भैकुराटो ग्रन्थः, जालूकः ॥
Why do we say 'a book'? Observe तक्षकृतः प्रासादः 'the palace made by Taksha'.
That which one originates is called कृतः and is therefore an 'art', while knowledge of laws &c. that already exist, but are discovered for the first time by some one, is called उपज्ञातं ॥,