2. S.S. Varma (tr. Danielle Audouin), Yama et Niyama, Yoga Pratique
I believe this was originally published in India as "Yoga Practice: Yama and Niyama"; I read the French Kindle edition, but various Indian websites seem to have copies of the original available. It is an introduction to yama and niyama, which are often referred to as the ethical foundation of yoga. It gives the best explanation of these two concepts I've seen, with a clear account of what the difference is supposed to be (the traditional translations of "restraints" for yama and "observances" for niyama never quite worked for me as a way of making sense of which practice belongs on which list). It also has some useful practical suggestions for incorporating them into your daily practice.