Normally I like reading the original text, rather than the books written later-on on the subject. Now-a-days I am practicing Yoga and hence was looking for ancient literature on yoga. I found this URL useful for knowing about the original literature -
Hatha Yoga Literature
It tells about following five texts -
1. Hatha-Yoga-Pradipika - the monther of all texts on hatha yoga.
2. Goraksha-Shataka - probably the second most important text
4. Yoga-Sutra and Yoga-Bhasya - mostly deals with raja yoga; this is the text from where all hatha yoga text is drawn
5. Sat-Chakra-Nirupama - mostly deals with chakra awakening
Please let me know in case I am missing some important text here.