To get Author’s additional details we may need to expand the selected item as show below.

https://server/sites/sitecollection/_api/web/Lists/GetByTitle('Documents')/Items?$select=Author/Title,Author/Name,Author/EMail,Author/MobilePhone,Author/SipAddress,Author/Department,Author/JobTitle,Author/FirstName,Author/LastName,Author/WorkPhone,Author/UserName,Author/Office,Author/ID,Author/Modified,Author/Created,*&$expand=Author

Following list show all the valid Author fields

1. Title
2. Name
3. EMail
4. MobilePhone
5. SipAddress
6. Department
7. JobTitle
8. FirstName
9. LastName
10. WorkPhone
11. UserName
12. Office
13. ID
14. Modified
15. Created

Source: https://social.technet.microsoft.com/wiki/contents/articles/31210.sharepoint-2013-get-user-details-from-person-or-group-field-using-rest-api.aspx

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