best app to use for a macbook pro to write a book

Is this a book to be printed or an ebook? If an ebook, where do you want to sell it?

If you are talking about an ebook to be sold in Apple's iBookstore, then the best app is probably iBooks Author. Other apps common used are Pages and InDesign.

Who will you be asking to publish the book? If they prefer Word format, then it only makes sense to use Word. If they want PDF, there is a large range of word processors you could use -- Word, Pages, Nisus, etc.

There's a free clone of Word you might want to try -- LibreOffice.

You might want to check the advice at Lulu.com, in case you wind up going that route.

http://www.lulu.com/create/books