But it is rather good. Fortunata and Jacinta are two women in 1870s Madrid who both love Juanito Santa Cruz, the scion of a dynasty of clothing magnates; Fortunata is working class and bears him a child; Jacinta, his cousin, marries him by a family arrangement which becomes largely a love match. Most of the book is about Fortunata's ups and downs as she bounces from man to man, Santa Cruz always in the background, and Jacinta vaguely and uneasily aware of her rival.
Pérez Galdós is often compared with Dickens, but I think he's more in the line of the great Russian novelists - he is not trying to be even a little bit funny (none of the characters are simple caricatures - even his belching priest displays a deep insight in one important chapter). He is also very much engaged with both high and low politics - Spain in the early 1870s had a lot of regime changes (I had no idea!) and also Santa Cruz's exploitation of Fortunata is surely intended in part as metaphor for the class struggle. She is certainly the most interesting character in the book, but there are plenty of them.
Anyway, it is rather long, but I felt it worth making the effort in the end. I see that the same author's Compassion is on this list, so I may even give it a try some time.