In the whole of the Tibetan Buddhist tradition, there is no single treatise more deeply revered or widely practiced than A Guide to the Bodhisattva Way of Life. Composed in the eighth century by the Indian Bodhisattva Santideva, it became an instant classic in the curricula of the Buddhist monastic universities of India, and its renown has grown ever since. Santideva presents methods to harmonize one's life with the Bodhisattva ideal and inspires the reader to cultivate the perfections of the generosity, ethics, patience, zeal, meditative concentration, and wisdom.
This is one of my favorite books of all time, and I read it every couple of years. A Bodhisattva is a person who puts aside an attempt to attain nirvana, a personal form of peace, and instead focuses on helping others. Whether this is your intention or not, there are still so many good principles in this book to follow, in order to live a more content life. It is a lot like the Tao Te Ching, in that it can be read constantly in life, and new meaning keeps coming out of it. Sometimes a little over the top with some dogma, but as a book likely composed from multiple authors, that would be hard to avoid. There will be parts that speak to you; follow those.