How can I find the CD related to a MP3 Album?

You can use the RelationshipType parameter with the RelatedItems response group to find the CD related to a specified MP3 album download.

For example, consider the album Human by The Killers. The ASIN of the MP3 version of the album is B001L8LCXY. To find the CD of the same album, do an ItemLookup on the MP3 album's ASIN (B001L8LCXY) with RelationshipType=AuthorityTitle:

>>> View ItemLookup for B001L8LCXY

In the RelatedItems element, you find the ASIN of the parent - B001HCK0H8.

Do basically the same ItemLookup on the parent ASIN (B001HCK0H8):

>>> View ItemLookup for B001HCK0H8

In the RelatedItems element you'll see the ASINs of CD, LP, and MP3 versions of the album. The web service lists a maximum of 10 related items per query. If the query results in more than 10 related items, add the RelatedItemPage parameter to your query and set it to "2" to return the second page of ten related items, "3" to return the third page, etc.

One of the ASINs in the RelatedItems element is B001FWRZ46. This is the CD version of the album.