According to my handy OED, "epistolary" means "of or pertaining to letters or letter-writing" and was first used c. 1656. Presumably it has its origins from the much older word "epistle," which harkens back to Latin and Greek. The part of the word's definition that I found most interesting was:
Chiefly (from its use in translations from L. and Gr.) applied to letters written in ancient times, esp. to those which rank as literary productions, or ... to those of a public character, or addressed to a body of persons. In application to ordinary (modern) letters now used only rhetorically or with playful or sarcastic implication [2d ed.].Clearly, we need to bring back "epistle" into common parlance, as well as celebrating the epistolary novels that have sprung from it. Some of the earliest novels were done in the epistolary style--Clarissa (1748) and Evelina (1778) were both mentioned last night--but they aren't the kind of books that non-English majors are likely to pick up. Slightly more modern epistolary novels such as Jane Austen's Lady Susan (unpublished until 1871) and The Moonstone (1868), by Wilkie Collins, might appeal to Austen fans and mystery buffs, respectively. Someone interested in venturing across the Channel might consider Les Liaisons Dangereuses (1782), by Pierre Choderlos de Laclos. But while the epistolary novel was common in the 18th and 19th centuries, it's rare to find one published recently.
The format of the epistolary novel presents several challenges to a modern author. The action can't really be immediate, and the plot must be conducted through the medium of letters. Letters are not generally written in haste (or at all, these days), and the characters have to be the sort who would be writing in the first place. In a world where instantaneous communication is more and more common, and it's difficult to imagine having to wait weeks or even months to receive a letter, crafting an epistolary novel can be challenging. Most authors looking for a similar feel would probably opt for the diary or journal format instead. So, what are some more modern epistolary novels that might be worth reading--after finishing up Guernsey, for example?
The book I turn to immediately is not strictly a novel, but it does fit the tone and subject of The Guernsey Literary and Potato Peel Society beautifully: Helene Hanff's 84 Charing Cross Road. Published in 1970, the book captures an exchange of letters between Hanff and the employees of a London bookstore. The book is short, sweet, funny, and heartbreaking all at once.
Alice Walker's The Color Purple (1982) is an epistolary novel, as is Richard Wright's Clara Callan (2001). For young adult readers or Regency romance fans, I recommend Sorcery and Cecelia or The Enchanted Chocolate Pot (1988), the first in a series by Caroline Stevermer and Patricia C. Wrede. Other recommendations from Twitter users included:
A Woman of Independent Means (1978), Elizabeth Forsythe Hailey
Griffin and Sabine (1991), Nick Bantock
Ella Minnow Pea (2001), Mark Dunn
Gilead (2004), Marilynne Robinson
Which Brings Me to You (2007), Steve Almond and Julianna Baggott
For more excellent recommendations, visit The Guernsey Literary and Potato Peel Pie Society website, bearing in mind that the definition of "epistolary novel" is a tricky one to establish. If we assume that an epistolary novel consists entirely of letters, then a great book like A.S. Byatt's Possession, in which letters are crucial to the development of the plot, would be left out. The Prestige, by Christopher Priest, also features an epistolary section.
And although it's a bit of a stretch, the Internet Girls novels (Ttyl, Ttfn, and L8r, g8r) by Lauren Myracle do have an epistolary flavor, and could indicate something about the future direction of epistolary works. In 2007, a Finnish novel was published that consisted entirely of text messages between a businessman and his friends and relatives. Twitter itself features an asynchronous and "@" communication that could serve as epistolary shorthand, and there are already various Twitter-based literary projects. If anyone else has more suggestions (or corrections, I haven't read all of the novels listed here, although I did page through the ones on the shelf in my library), please leave them in the comments.
ETA: Richardson's Pamela, because Donna pined for its inclusion in the list. Which means that I should also include Shamela.