You don't need a personal relationship with the author but it helps to have a personal relationship with the book.
That book on my SBI! site converts very well because I wrote a review and actually bought the book. A lot of people just slap a book graphic on their site and say "It's a great book and buy now."
But it helps to actually show you've read it and tell your audience what you can learn from it. I found the book on Clickbank about a year ago, bought it, read it, reviewed it and Voila.... it brings in a few hundred dollars per month in revenue.
Check out
www.clickbank.com and do a search for books in your niche. Their commissions are high too because the books are digital. The book I recommend on my site pays about $27 per order.