Books are usually hand kerned to avoid large, variable width spaces that are hard to parse. Books have complete control over how the content is formatted and displayed.

Yes (at least all the professionally published ones I've seen). This is a major reason why I prefer ebooks.

Most books left-justify text.

Huh? I have about a thousand books in my house. Only a handful have left-justified text.

Gutenberg’s Bible was set fully justified in the 15th century, and that has been the overwhelmingly predominate practice in printed books from then to now.