Throughout literary history, novels have varied in length, depending on both commercial concerns and artistic aspirations. There are your basic epics like Tolstoy's War and Peace (about 560,000 words in the English translation) and Margaret Mitchell's Gone with the Wind (roughly 423,575 words); your much slimmer novellas (or “short novels”) like Steinbeck's Of Mice and Men (29,160), Hemingway's The Old Man and the Sea (approx. 27,000), and Capote's Breakfast at Tiffany's (26,433); and then your average length books, falling somewhere between 60,000 and 110,000 words (Salinger's The Catcher in the Rye clocks in at 73,404 words, for example). This article lists many more famous titles with sometimes surprising word counts.
Scholars of the subject vary in their opinions of what length to shoot for, if any. The general expert consensus is your book should be at least 70,000 words to be considered a major league contender, but not more than 100,000 if you're an unproven commodity. Those same industry observers also acknowledge there are many legitimate variables like the subjective art of storytelling which may alter the outcome. The key to success has more to do with content than volume – quality over quantity.
Still, as with any creative endeavor marketed for purchase in a busy field, you should assume your effort will not be the exception, even if you're promoting your product as wholly unique. Many authors who choose to publish only digital versions of their books are content with writing “novelettes” which can be as little as 10,000 words, up to 50,000 words (the rough tally of Fitzgerald's The Great Gatsby and Vonnegut's Slaughterhouse-Five). When sold at only 99 cents, these briefer efforts can prove both lucrative for the author and a better deal for the reader, in terms of expense as well as time.
As with many aspects of the modern publishing industry, the DIY digital revolution has changed many of the rules simply by breaking them, opening the market for authors who can tell their stories more economically without the pressures of big publishing companies demanding “beach books,” as well as longer-winded scribes who, thanks to new technologies - whether it's eBooks or POD (Print On Demand), don't have to worry about killing too many trees with their seemingly endless page-turners.
Some general guidelines when editing your book:
As a reader, how short is too short, how long is too long, or what is just right when it comes to your ideal eBook-length?