Looks most likely postural,
Make sure your book is at slightly higher level then your desk.
You can have a plain muscle relaxant for it and a low dose of pregabalin 50mg can help you suffix the problem.
Next Steps
correct your posture, use a chair with neck support and also use a very small pillow while sleeping
Health Tips
You can use books elevator,like you see them in government officers desk, that will help dramatically.
Consult an orthopedic specialist if you don't see changes after 10 days