Literature thrives best when it is half commerce and half art.
Literal
Literature [topic-は] it [subject-が] half business, half art is [when-とき] most flourishes.
半ば…半ば… is a balanced 'half X, half Y' construction. 栄える (さかえる) means to flourish or thrive. The whole sentence is a literary aphorism about the commercial and artistic nature of writing.