do you mean software to create ebook (.pdf)? if so, adobe acrobat. but i think you can write your ebook in word and convert it to pdf. i believe there are free programs that can do that. i also heard about software called foxit or something.
foxit looks good. I'm going to download the free trial and see if it is what I'm looking for. Santacruz
The open office word processor also has a super easy convert to .pdf function and it's completely free so no trial stress.