And different tools are suitable for different types of books.
Some books rely on very specific layout and typography. Then you obviously need some tool that can handle that. Indesign has already been suggested. Scribus is another possibility. But if you don't have a lot of previous experience using this type of tool, you should perhaps let someone more experienced handle the layout.
On the other end of the scale is a typical novel. Just text, paragraphs and chapters. Then just a simple text editor would be enough, perhaps one text file and per chapter. And you could use some form of markup to quickly create a pleasant and consistent layout with headings and so on.
You can start with a set of plain text files, one per chapter. Perhaps written using some type of markup.
Then convert the text files to html, using the markup tags.
After that import the html to sigil and create a epub.
Either way you should start with a very small test project and try to set up an environment that allows you to test and learn all the tools.
Here is more information about "markdown", a popular markup system that allows you to write the book as a set of plain text files, and with minimal effort convert them to html that acan be imported to sigil.
http://daringfireball.net/projects/markdown/
And here a slightly more advanced version, "Multimarkdown" :
http://fletcherpenney.net/multimarkdown/