Hello Oliver,
Contacts is also a BusinessPartner. You can retrieve the name as shown below
var Contact = BusinessPartner.Retrieve(ContactID); //ContactID is of type BusinessPartnerInternalID
var ContacCurrCommon = Contact.CurrentCommon;
var FormatName = ContacCurrCommon.BusinessPartnerFormattedName;
var FirstName = ContacCurrCommon.Person.Name.GivenName;
...
If you need more information on the BO and Specific fields, you can open the respective UI (ex Contacts TI) from UI designer and check the UI binding to Data model and BO model.
Thanks Pramodh