if you want to make epub books, i recommend using the online interface at
feedbooks (unless you prefer to handcode, which is of course also an option). if you don't want to publish your book through their distribution plateform, just make the book and then request a preview which you can save to your computer. feedbooks can also create a mobipocket file so you don't have to do the work twice.
edit : take a look at their
help pages which will explain some best practices for publishing epub books, and give you some useful info about book structure.