Use an online resource such as Ralan.com to locate book publishers who publish in e-book format (see "Resources"). Ralan is a comprehensive list of publishers that gives detailed submission information on how to submit your manuscripts to many book publishers. Ralan also gives payment information and publishing format information. There are a number of e-book publishers listed here.
Visit the Duotrope Market Resource guide. Duotrope is a comprehensive market listing that allows you to search markets by custom criteria. Choose electronic from the drop down menu and you will be presented with a list of publishers who produce e-books.
Use eXcessica if you have short erotic stories or novels. eXcessica publishes your books in e-book form, then distributes them on their website as well as several others. eXcessica pays royalties through PayPal for e-books sold on their website and by check for royalties generated through other sites.
Consult the directory of e-book publishers (see "Resources") for a comprehensive list of publishers who only publish books electronically. This site is also useful for information related to publishing books electronically and features many helpful tips for writers.