I hope this isn't a criticism. Referral links are about the only reasonable way to monetize a blog, and are a whole lot less intrusive than advertising.
Fundamentally, I don't have any problem with it. If you're going to link to something anyway, as would be obvious with a book list like this, it's a no-brainer to use affiliate links, get a cut, and earn back some hosting fees. The price is what it is for the consumer, whether they get there through an affiliate link or not.