Complex sentence: Blackie and Son initially published books sold by subscription, including religious texts and reference books, but later published single volumes, particularly educational texts and children's books, taking advantage of compulsory education from 1870.
Split into simpler sentences: Blackie and Son initially published books sold by subscription, including religious texts and reference books. Later the firm published single volumes, particularly educational texts and children's books, taking advantage of compulsory education from 1870.