You can publish your ebook in .pdf format as well. In my opinion, pdf format is more useful and easier to create.
High number of software could be used for writing the book and then it can be easily convert to pdf format.
For example, OpenOffice Writer could be used for writing the book and then it could be export to pdf from the OpenOffice. (OpenOffice Writer very powerful word processor and the program is free, so this is another advantage)
PDF is a poor choice for ebook publishing because it uses a fixed page layout rather than reflowable text. If you look at ebook readers like iBooks or Kindle, they reflow the text to suit the screen size and orientation, as well as the user's viewing preferences, resulting in a much more comfortable experience for the reader.
With PDF, your book uses fixed size pages regardless of the screen size, and the user either has to zoom out to see the whole page (and squint as necessary), or zoom in to make the text big enough to read and then scroll left and right for every single line of text, which is not at all a good experience.
You should use epub (which is compatible with iBooks).