Interestingly, they seem to be a bit easier to sell than ebooks. Clearly, people are still very fond of sitting down with a good old fashioned tome and flicking through the pages.
But traditional books can be expensive. Which is why online booksellers that offer them cheap, with preferably free shipping, are the most popular. Book Depository is one of these. It has a huge range of books with free shipping worldwide and a good affiliate program. Have been paid so I can vouch for it.
Blogs are ideal for book promotion
Blogs are a great way to promote books. Probably the most effective method is to write reviews of them and include your affiliate link in the post. Needless to say you should disclose the nature of it, and that if people buy it you'll receive a commission. And while it's best to review books you do generally enjoy, make the reviews balanced. Blog readers will appreciate a thorough, fair review that includes a bit of criticism. Such appraisals won't reduce the odds of them buying the books subsequently. Might even increase them, actually.
While reviews are great, they're not crucial. You can just link to specific books in blog posts. As long as they're relevant to the content and good quality, people will click on those links from time to time. And some will end up buying.
Social media is a good fit too
Social media is also a good place to promote specific books. But you should do this sparingly, and only after you've built up a substantial, engaged following.
Twitter is ideal for this. Book titles tend to fit snugly into the 140 character limit. They also often have a lot of good keywords that you can turn into hashtags to make them more findable. And as long as you're not constantly posting them your followers will appreciate being pointed towards informative, relevant, interesting reads.
If you make a habit of engaging with your followers, then they're more likely to click on those links, too. That's because they trust your opinion. So, respect that trust, and only promote books that you have read or are pretty confident are good qaulity. If you just link to junk they'll stop clicking after a while. Some may even end up unfollowing you.
Authors love Twitter
You should also engage with the authors themselves. Again, Twitter is a great place for this. Most of them are on the site anyway, and many are influencers with huge followings. So, they'd be worth following, retweeting and mentioning in any case.
And if you do promote their books they'll be much more likely to notice you and follow back. They'll also retweet your links to their books from time to time. (This has happened to me a few time now.) Obviously they're motivated to do this for financial reasons. But it also reflects well on them. Your unprompted promotion of their work is social proof of their excellence in their fields, after all.