What is a good program to buy for your computer if you wish to write a book?

Are you asking about specific Novel writing software? That stuff is rather expensive, but the best, from the ratings, is Dramatica.

If you just need a word processor program, then OpenOffice writer is a solid free application.I've not used OneNote so I can't comment on it's use.

i will recommend ms office onenote,because you can easuly track all your thoughts with linked note(you can link any page within this program),onenote supports handwriting,voice recording,also you can place any selected text (or complete page with pictures)from internet in any page in onenote(possible only from internet explorer)