You have asked a great question. I wish there was a one size fits all answer. There isn't.
First of all, Reunion is probably not the tool you want to use to produce a book that has a family history narrative. You will use Reunion in that process, but it will most likely be one of several apps you use.
I think the first thing you need to do is decide what form you book will take. If it will be printed you might want to consider using www.lulu.com. They do print on demand books from PDFs you supply which most any word processor will produce. Then it's just a matter of deciding which app is right for you: Word, Pages, Mellel, Libre Office, etc. I wrote about my experiences publishing a book last with Lulu last year. You can read that at http://www.reuniontalk.com/showthread.php?t=6847.
If you are going to be citing your sources then you need to find software for that which works with your word processor. Someone mentioned Zotero (https://www.zotero.org/) on here the other day. It seems to be good stuff.
You should check out Edgar Dohmann's site. He has a page posted where he tells about a book he wrote for his family. It has some sample chapters available for download in PDF. The URL is:
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