Our experience with program directors, the most successful applicants are those whose letters of recommendations are good enough to make the staff rea them till the end. There are some guidelines that make your letters look up to code. These books will have samples of residency letters of recommendations with the guidelines to help you and your mentor write a competitive LOR that will help you earn more interviews:

Anesthesiology 
Combined Med Peds 
Dermatology 
Emergency Medicine 
Family Medicine
Internal Medicine
Neurology
Neurosurgery 
Obstetrics & Gynecology
Orhtopaedic Surgery
Pathology 
Pediatric
PM&R
Psychiatry
Radiology
General Surgery