it doesnt, google picked up the links to your other pages through your sitemap/menu.

linking to other pages will just confuse readers and temp to click - unless that's your purpose?

This is the technique that get wikipedia ranked so high in search engines. Interlinking is a good practice.

i don't think it has any valuable SEO purposes. the text will be in your book, rather than pages indexed in google. hence, no authority or relevance would be built by making those links.

for SEO purpose, it does help, each link to another page, you are passing link juice to it, so do it in moderation, or that page will lose it's juice..

Yes, it helps, just remember to remove the "nofollow" tag.