Vertigo: Although it may seem a bit too much to take on, you might find Burke's essay "A Philosophical Inquiry into the Origin of Our Ideas of the Sublime and Beautiful" as a good source to see some of the deeper, more "terrific" aspects of this and other "horror" novels of the same period. Using Burke's schema, Frankenstein (and in fact the majority of the Gothics) is closer to the "sublime", which Wikipedia rather succinctly sums up as:
In short, the Beautiful, according to Burke, is what is well-formed and aesthetically pleasing, whereas the Sublime is what has the power to compel and destroy us.
Their summing up of the major ideas of the essay are as follows:
The origins of our ideas of the beautiful and the sublime, for Burke, can be understood by means of their causal structures. According to Aristotelian physics and metaphysics, causation can be divided into formal, material, efficient and final causes. The formal cause of beauty is the passion of love; the material cause concerns aspects of certain objects such as smallness, smoothness, delicacy, etc.; the efficient cause is the calming of our nerves; the final cause is God's providence. What is most peculiar and original to Burke's view of beauty is that it cannot be understood by the traditional bases of beauty: proportion, fitness, or perfection. The sublime also has a causal structure that is unlike that of beauty. Its formal cause is thus the passion of fear (especially the fear of death); the material cause is equally aspects of certain objects such as vastness, infinity, magnificence, etc.; its efficient cause is the tension of our nerves; the final cause is God having created and battled Satan, as expressed in Milton's great epic Paradise Lost.
It is not by chance that one of the most famous quotations cited within Mary's book is that bit from
The Rime of the Ancient Mariner:
"Like one that on a lonesome road
Doth walk in fear and dread,
And having once turned round walks on,
And turns no more his head;
Because he knows a frightful fiend
Doth close behind him tread."
Incidentally, that swinging back and forth you mention touches on another common interpretation of the novel: that the creature and Frankenstein are two sides of the same character; the "monster" he creates is Frankenstein's own shadow, as it were, as Hyde is Jekyll's.
On this type of "horror" (and I use the quotations marks for a reason), one might keep in mind Poe's comment when he was criticized for being "too German" in his terror tales that "I maintain that terrors is not of Germany, but of the soul".
Should you be interested in a brief look at Burke's essay, the Wikipedia entry can be found here:
http://en.wikipedia.org/wiki/A_Phil...gin_of_Our_Ideas_of_the_Sublime_and_Beautiful
while the entire essay may be found here:
http://www.bartleby.com/24/2/
though you may also (or perhaps instead) want to take a look at Letitia Barbauld's shorter piece, "On the Pleasure Derived from Objects of Terror":
http://www.litgothic.com/PDFOther/barbauld_terror.pdf
Most of the terror writings of this period were written with this philosophical basis in mind.
As for my so denoting "horror"... I am here thinking of how we moderns confuse "horror" and "terror", which are two related but distinct things.* As Sir Devendra P. Varma (one of the leading authorities on the Gothic tale of terror) put it: "The difference between Terror and Horror is the difference between awful apprehension and sickening realization: between the smell of death and stumbling against a corpse."
As an elaboration, he explained:
Terror creates an intangible atmosphere of spiritual psychic dread, a certain superstitions shudder at the other world. Horror resorts to a ruder presentation of the macabre: by an exact portrayal of the physically horrible and revolving, against a far more terrible background of spiritual gloom and despair. Horror appeals to sheer dread and repulsion, by brooding upon the gloomy and the sinister, and lacerates the nerves by establishing actual cutaneous contact with the supernatural.
Once again, Wiki proves to be helpful in summarizing these matters:
http://en.wikipedia.org/wiki/Horror_and_terror
If all this seems a bit rarefied and theoretical... it may well be; but understanding such a background
and the implications of what these distinctions are may help one to appreciate much more fully the experience of reading these types of works.
*For those interested, here are the comments of several others making that distinction:
http://academic.brooklyn.cuny.edu/english/melani/gothic/terror_horror.html