Alice in Wonderland: Whether you are partial to the Disney cartoon or the Johnny Depp/Tim Burton version is your cup of tea (pun intended), this theme is very flexible. Besides having a mad tea party, you could also hold the party in the Queen of Hearts's court, complete with jam tarts and red (and white) roses. This is also the perfect theme if you want to be a little quirky/eccentric, since Wonderland is many shades of crazy. As an added bonus, Alice in Wonderland has a ton of characters, so this gives your guests a range of characters to dress up as. If you need more inspiration, Lenny's Alice in Wonderland site has a lot of excellent party ideas ranging from the menu to the party favors.
The Great Gatsby: If you are looking for something with a more adult theme and you want to give your guests a chance to dress to the nines in fabulous 1920s regalia, then a Great Gatsby-themed party might be right for you. If you want to pay homage to one of Jay Gatsby's parties, then break out the champagne, decorate everything in art deco, and be a little (or very) ostentatious. For something a little more low-key, you can replicate the reunion tea that Nick had at his humble abode, which featured the usual suspects in terms of an afternoon tea. Party Remedies also offers some easy ideas.
The Lion, the Witch, and the Wardrobe: This is another child-friendly theme, and I could see it being a lot of fun for a neighborhood party. Besides decorating your front door to look like a wardrobe, you could take advantage of anyplace that has their Christmas decor out by transforming your party room into a winter wonderland (since Narnia is in perpetual winter until Aslan returns). You could serve Turkish delight as well as tea party foods (what is it with all of these books and tea?). eHow.com has a surprisingly elaborate list of ideas that can make your Narnia party one to remember.
If you don't want to commit to just one book, you can throw a party with a specific genre as the theme. This could be particularly fun for several reasons. Besides utilizing all the cliches of the genre, this theme is a little more open-ended, so you have a little more wiggle room in terms of what the party can include and guests can be more creative when it comes to costumes. In terms of genres, the possibilities seem endless, but here two of my favorites:
A (bad) romance-novel themed party: Even though there are some good romance novels out there, most people are more familiar with the stereotypes associated with the genre. This can allow you to let the party be delightfully tacky and over-the-top. For the decorations, you can go crazy with boldly colored pillows, flowers, and candles. Or, if you want more of an exotic theme, try to go for the harem look with a tent and vaguely Middle Eastern decorations (authenticity is not the point - this is a bad romance party). The food should be finger food, or you could go a different route and serve things that are insanely suggestive (or completely in your face).
Just for fun (and because you can probably go to a used book store, library book sale, or flea market and buy these by the ton), use Harlequin Presents romances as party favors. Honestly, what is a better icebreaker than letting your guests look at books with titles like Powerful Greek, Housekeeper Wife? You could even hold a contest for the most ludicrous passage in a book.
A mystery: Besides the ubiquitous murder-mystery theme, you could go a little more PG and just have a regular mystery for the guests to solve. Set the mood with a dark, spooky atmosphere in the house. Invite guests to dress up like their favorite sleuths (Nancy Drew, Sherlock Holmes, and Miss Marple come to mind), and have them solve puzzles to earn more clues.