。
She finished writing the book with little effort.
Literal
[topic-は] meager effort [with-で] book [object-を] finished-writing.
書き上げる is a compound verb where 上げる ('raise up') functions as an aspectual marker meaning 'finish, complete' — productive across many compound verbs (作り上げる 'finish making,' 仕上げる 'finish up,' 描き上げる 'finish drawing'). The で on 努力 is the instrumental, marking the resource (effort) used to produce the result. わずかな努力で sets up an implicit contrast: typically writing a book takes enormous effort, so finishing one with little effort highlights talent or ease.