It is a long time since I last read this, and I had forgotten a number of interesting points: that Frankenstein's original betrayal of his creature is right after he animates it, when he runs away; that Frankenstein attempts to create a female version of it in, of all places, Scotland (the Orkney Islands, to be precise); the interlude with Felix, Agatha, Safie and the old man in Germany; the detailed account of the geography of the surroundings of Geneva and to a lesser extent England (compare the much vaguer descriptions of Ingolstadt and Ireland); and the ending - I had the idea that the creature fled to a hidden Arctic city, but perhaps I was thinking of Aldiss's Frankenstein Unbound.
I had also forgotten how much dialogue Frankenstein's creature gets. This is really important because the story is about families, and the creature's isolation when rejected by Frankenstein, its 'father', resulting in its murderous resentment. It encounters two sets of extended families in the story - the Frankensteins, and Felix and Agatha's household - and is excluded, cruelly if for understandable reasons, from both. (NB the the framing narrative is also a family one, letters from Walton to his sister.) No wonder its sole demand of Frankenstein is to create a soul-mate. I'm not aware that any cinematic adaptation manages to generate as much sympathy for the creature as Shelley's original.
The other key figure is of course Frankenstein, who starts off as a keen if naïve scholar, unifying old lore and new science to create wonders, and then spends the rest of the book wrestling with his conscience over the consequences of his actions. Although he is the first-person narrator for most of the book, and unlike the creature doesn't actually kill anyone, it's much more difficult to feel sympathy for him as he tries to evade his responsibility, both to his biological family and to his creation.
I should love to read a biography of Mary Shelley, to see where she got these fairly vivid models of human misbehaviour from. (Wikipedia suggests it was direct observation of Percy, but that seems too easy.)
Anyway, it's a quick read, and in places a surprising one, and if you have any interest in the sf genre it is a hugely important book.