I use Amazon almost exclusively for buying hardcover Dungeons & Dragons rulebooks, simply because the savings is so dramatic compared to the cover prices charged by my local bookstore (for example, Amazon charges $19.70 for a book with a cover price of $29.95). As a result, almost everything they recommend is D&D-related.
Has anyone noticed tie-ins with other products, like direct mail? I ordered two children’s books thru Amazon – the only order in at least a few years of children’s materials from any vendor at all – and ever since I have been getting complimentary copies of Child & Parent magazine. I’m pretty sure the source was Amazon.