You can obtain a contact's detailed information by using the GET method on a contact resource, which would look like:
https://api.constantcontact.com/ws/customers/{username}/contacts/{contact-id}
If you do not have the URI for a contact, you can obtain one from the contacts collection and finding the contact's <entry>. Within <entry> element, there is a <link> element with attribute rel="edit". You need to take the value of the <link> element and construct the URI that looks like the above.
This request returns the XML body that describes a contact's full detail, the information includes contact details as well as contact lists to which the contact belongs.