The Merck Manual of Diagnosis and Therapy is one of the best-selling medical books of all times and has a section dedicated to Musculoskeletal and Connective Tissue Disorders.
Wheeless' Textbook of Orthopaedics is provided by The Duke University Medical Center's Division of Orthopedic Surgery and is a vast and continuously updated online orthopaedics textbook, with more than 11,000 pages and 5,000 images.
The National Institute for Health and Care Excellence (NICE) Guidelines. NICE makes recommendations to the NHS on new and existing medicines, treatments and procedures and on treating and caring for people with specific diseases and conditions.
United States specialty societies representing more than 500,000 physicians developed lists of things physicians and patients should question in order to improve care and eliminate unnecessary tests and procedures.
The Geneva Foundation is an amazingly well organized directory of links organized by diseases and conditions mostly on gynecology and obstetrics. There is also an excellent link directory on free medical journals in all specialties, including Rheumatology journals.