For more information on myofascial pain, see the following references:

Myofascial Pain and Dysfunction: The Trigger Point Manual by Janet G. Travell, M. D. and David G. Simons. available from Lippincott, Wiliams & Wilkins at lww.com. This two-volume work is considered to be the Bible of muscular pain. If you can't read medical terminology, take it to your doctor. If he doesn't have his own, well-marked, dog-eared copy with lots of yellow stickies, paper clips, stars, arrows, underlinings and highlighting in four different colors, he probably doesn't know muscular pain. (Pharmaceutical brochures are no substitute and muscle relaxants do not relax trigger points.) These are medical textbooks costing around $100 each. You should be able to find them in any good medical library or through inter-library loan.

Fibromyalgia and Chronic Myofascial Pain by Devin Starlanyl, M. D.
Doctors have long dismissed fibromyalgia as a women's disease ("and these women are CRAZY!") That is starting to change, in part due to the development of far too many tests that reveal clear clinical abnormalities behind the pain, and in large part due to Starlanyl who is a doctor herself -- and suffered from severe fibromyalgia. Fibromyalgia is still commonly confused with myofascial pain syndromes. (Nearly every long-time beautician I have checked has upper back and neck pain with severe trigger points in her trapezius -- and a diagnosis of fibromyalgia). You can also suffer myofascial syndromes with the beginnings of fibro, or have fibro with myofascial syndromes and trigger points. To clarify the difference, Starlanyl's book includes the superb trigger point and pain referral diagrams from Travell & Simons' Myofascial Pain and Dysfunction. See also Devin's website for more information.

The Trigger Point Therapy Workbook by Clair Davies.
Davies was a piano tuner nearly incapacitated by shoulder pain. Ultimately, Travell & Simons' myofascial techniques were the only thing that helped and what ultimately resolved the problem.
(I was a field geologist nearly incapacitated by migraines -- in part, I suspect, due to neck trauma from the poorly fitting backpacks and equipment of 30 years ago, designed for Tall People only. Hot tip: NEVER EVER wear a pack that forces your neck into a head-forward position!) At any rate, Davies wrote the book I wish I had written when I first studied myotherapy, hoping to relieve my pain, 20 years ago.)
This book is solidly based on Travell & Simons, but emphasizes self-treatment (rather than clinical) techniques. If you lack medical background, use the Davies book with its clear non-technical English in conjunction with Travell & Simons for its terrific illustrations.