It's very hard for digitally published books to be especially successful. Digital publishing offers an avenue for books that have problems with traditional publishers. These are, probably in order of importance -
1) Name recognition - Publishers have trouble marketing someone who has nothing to commend them, either in past experience (ex-soldier/sailor/airman in the case of books about war), or present position (academics).
2) Subject matter - You can tackle name recognition if your subject is sufficiently broad to have wide appeal, especially into foreign markets. Foreign sales almost equal free money for publishers, although this may change in the next ten years, if it hasn't already.
3) Length - Writers often neglect the fact that traditional publishers think in certain sizes. This is partly a pricing issue, as the cost of publishing a book goes down as it gets bigger (until it gets too big), which feeds into the idea of price points, where potential purchasers might be put off buying because the book seems expensive for its size.
This book on the experience of Canadian soldiers during the 1941 battle of Hong Kong has achieved some critical praise, but really falls foul of all three of my rules above. Digital self-publishing was really its only route to getting into print.
At that point, though, the marketing of the book takes over. This is the hardest hurdle to leap of all, which is why it's probably better to exhaust all avenues of small publishers before going the digital route. A small publisher will at least have some ability to claim review space, where self-publishing has none.
Basically, unless you can build up a network of promotion via blogs and pushing your product on online forums, you've got little chance of sales outside of your immediate market of friends and family.