Surya Upanishad or Surya Upanishath is a part of Atharvaveda. Here you can download Surya Upanishad PDF document for free.
Surya Upanishad is the most important sacred text to know about Surya Bhagavan. The Upanishad explains the legends and significance of Lord Surya in a very brief way.
It gives an introduction to Dwadashaditya (12 aspects of Sun).