Amazon.co.jp is the Japanese version of Amazon. It is similar to Amazon US in that it sells pretty much everything, and at (usually) competitive prices. Many books, figures, dvds, and more are available there, usually even cheaper than you would find at US retailers, or sites like AmiAmi.
What might not be obvious is that much of their site is actually available in English! All you need to do is change the language using the top menu.
From there you can easily make an account (you will need to make a new account, since it operates separately from Amazon in other countries).
One of the best benefits of buying from Amazon Japan is AmazonGlobal shipping, which ships to North America in only 3-4 days (!!) and for very fair rates. The packages are delivered in the US using the DHL courier service. This is much faster than you would get ordering from most other online retailers like AmiAmi, CDJapan, etc. that will ship from Japan with SAL, and it's even faster on average than EMS.
In my experience their handling and packaging are top notch as well. I have yet to see any soft of dent on tens of orders I've made, so DHL is reliable in that regard. And if ordering multiple items, they will usually end up shrink-wrapping and affixing to a piece of cardboard to make sure the items arrive safely.
Overall, Amazon Japan is highly recommended as one of the best ways to buy almost anything from Japan!