Lady Macbeth from Macbeth - Unlike some characters, there is no single, indelible image that is Lady Macbeth, which can be a bonus (even if it does require that you explain your costume to people throughout the evening). For Lady Macbeth, you can go with a more formal, regal look (think velvet gown and elaborate hair) or a more severe ensemble (whenever I picture Lady Macbeth, Judy Dench's version comes to mind). Perhaps her most recognizable outfit is the nightgown associated with the sleepwalking scene, which would allow you to be very comfy (layers would be a good thing) and mutter "Out, damned spot! Out, I say!" all evening.
The Girls (Bridget, Carmen, Tibby, and Lena) from The Sisterhood of the Traveling Pants - This idea works best if you have three female friends who want a group costume but no one wants to spend a lot of time or money coming up with one. The main thing each person needs is a comfortable pair of flattering jeans. After that, personalize the looks based on the different characters. Bridget needs long blonde hair (or, if you want to go for the second book, poorly dyed black hair), a soccer jersey or flirty top (when she's trying to get Eric's attention), and maybe a soccer ball. Carmen is trendier and curvier than her friends. Tibby might have a blue streak in her hair, a Wallman's (or Walgreen's) smock/vest, a nose piercing, and a somewhat sour expression. Lena dresses on the preppier end of the spectrum and tries to blend in.
The Narrator from "The Raven" - Our knowledge of the unnamed narrator of Poe's "The Raven" is limited, but he would be easy to recognize because of the raven. Given the late-in-the-day setting, slippers, pants, a shirt, and a nice robe would suffice. However, you would also need a stuffed raven to place on your shoulder (add the word "Nevermore" on a speech bubble coming out of the bird's mouth for further detail) or a friend to dress up as a raven to accompany you on your Halloween adventures. You might also have a picture of Lenore to look at and sigh over throughout the night.
Princess Elizabeth from The Paper Bag Princess - Elizabeth makes a refreshing change from the typical princess archetypes in their tiaras and fancy dresses. Additionally, she would be a (relatively) easy last-minute costume. As the book title suggests, she wears a paper bag instead of a gown and goes on adventures. Given the (probably) chilly temperatures and decency laws, I don't recommend that you only wear a paper bag. Instead, wear tights (or shorts or pants, depending on your preferences) as well as a tank top or long-sleeved shirt underneath your paper bag (or brown paper) dress. While Elizabeth's dress was simple, you could go all Project Runway with the idea and make your creation a more elaborate affair by styling your paper bag just so.
Scarlett O'Hara and Rhett Butler from Gone with the Wind - I can't believe that I haven't yet spotlighted Gone with the Wind here yet, but better late than never. I think that part of reluctant to include the book is that the movie, its costumes, and its overall look are so well known, but rather than run away from it, you could also embrace the iconography. Scarlett and Rhett (typically) require some planning, but the effect is well worth it. Depending on time, budget, and weather (seriously, running around in a hoop skirt and ballgown wouldn't terribly fun in 80 degree weather), you can go super-fancy in one of Scarlett's well-known, sumptuous ensembles, such as her green velvet dress (made from Tara's curtains), her red velvet party gown, or her picnic dress (Edelweiss Patterns has some beautiful pictures of a homemade version). To be a dapper Rhett, a period suit, preferably with tails and a vest, would do nicely. However, if you are short on time or actually want to fit into your car as Scarlett, you could go with her less famous but still iconic farmhand look by wearing a long, simple cotton dress and a determined if weary expression. Bonus points for defiantly saying, "As God as my witness..." throughout the evening. Rhett's dressed down look is even easier: all he needs is rakishly disheveled hair, long pants, and a white shirt (buttoning it up is optional).