Interesting read as a document of Victorian approaches to medieval art, but if you want a genuinely penetrating analysis of the arena chapel, I wouldn’t recommend Ruskin. He was primarily an aesthetician, and is thus completely uninterested in historical/theological backgrounds which, as sacred art, are vital. Also, the commentaries, written as they are for Victorian subscribers, indulge a view of Giotto as a sort of founder of modern art (Ruskin famously calls him an early bearer of the ‘flaming cross of truth’), and thus offers no insights as to just how saturated the frescoes are in medieval thought and culture (something on which more recent scholarship is generally in consensus)
TL/DR: if you’re interested in Victorian art history and the historiography of medieval art, read it. If you want to actually know what Giotto was on about, find something else.
For probably the most recent and most comprehensive art historical book, I’d recommend Henrike Lange’s “The triumph of Humility”, which will direct you to other more classic works in its bibliography as well