Complex sentence: They cover the period from 1805 to the end of the 19th century and provide glimpses of Pérez Galdós' liberal and anti-clerical views, which become more developed in the contemporary novels.
Split into simpler sentences: They cover the period from 1805 to the end of the century. We see glimpses of Benito's liberal and anti-clerical views, which become more developed in the contemporary novels.