To understand Freud's theory of sexual instinct, you can refer to the following works: The Origin and Foundation of Inequality Among Men 2. The Eudaimoniaonia of Health, Happiness and Love 3. The ideology of sex 4. Sabiens: A Short History of Humanity These are all important works by Freud, but it's important to note that these works are older works that may no longer be on the market. However, these books could be obtained by reading online or buying second-hand books. In addition, if you want to understand Freud's theory of sexual instinct, you can also refer to his other works such as The Psychology of Love and Dreams and Reality. As for the reading order, it is recommended to read in a temporal order so that you can better understand Freud's theory. You can also read the book "On the Origin and Foundation of Human Inequality" first. This book is one of Freud's important works. It is very helpful in understanding his theory of sexual instinct. Then you can read his other works in turn, which cover important areas of his research.