You receive free shipping if your order includes at least $35 of eligible items or $25 of books (in which case all other eligible items in the order also ship free). Any item with “FREE Shipping” messaging on the product detail page, that is fulfilled and shipped by Amazon, is eligible and contributes to your free shipping order minimum.
A while back, they raised it to $49, which I think slowed business a little.
This doesn’t effect me because I’m a long time Prime member! I just love having Amazon Prime! Read this post: Is Amazon Prime Worth It?!