Below are links to English translations of the Hatha Pradipika by Swami Svatmarama, a 15th-16th century yoga master. This work is considered one of the major classics of yoga literature. I prefer the first version offered and concur with the common-sense admonitions offered by the contemporary editor, "TK." The practices outlined in this 500-year-old text should not be undertaken unless you are under the guidance and strict supervision of a highly qualified yoga teacher. While the wisdom and instruction offered in this book is of tremendous value, many of the techniques and practices may not be suitable for contemporary practitioners, in my opinion.
In addition, I strongly recommend a short summary of highlights from this work compiled by Dr. Vijayendra Pratap in the slim volume, Secrets of Hatha Vidya (1996, SKY Foundation, Philadelphia, PA).