Can I include quotes in a company manual? How?

Yes, you can probably use the quotes with attribution since they can't be copyrighted or trademarked (in most cases). How you use the quotes though may raise issues. If you use the quotes out of context, such as using "This is the best product I've ever used! John Doe" to refer to your product instead of the one the author was really talking about, or if you place the author in a false light, such as "I felt such comfort in his hands. Reverend John Doe" when you're selling adult films, the author has every right to file suit.

Quotes from public domain sources may be used freely - these are typically over 100 year old. Current quotes - may need permission from the author or publisher - as attribution may not be enough. Ask for permission to be safe if the quote is not in the public domain.