Letters on Yoga, Vol.I by Sri Aurobindo
Letters on Yoga, Vol.II
Letters on Yoga, Vol.III
Letters on Yoga is a compilation of Sri Aurobindo's letters on all areas of spiritual instruction (see Integral Yoga).
Although he withdrew from public life and from meeting the sadhaks in person, Sri Aurobindo continued to instruct his students by letter, maintaining a voluminous correspondence, especially in the 1930s. Many of these letters were edited and then published. All the letters were revised by Sri Aurobindo before publication. Later these became three volumes of the collected works.
The Letters are light, easy to read, a delight. Of course this material has been selected from a huge selection of personal letters to disciples, with the irrelevant parts edited out. Once at the local Sri Aurobindo group (this was many years bag) one of the people who ran the group; (he was pretty old at the time) read out the entirely of a long letter that Sri Aurobindo had written him. Much of the letter went in the Letters (I recognised it at the time, but didn't note it, so i forget which one it was. He explained that he actually wrote to Sri Aurobindo about something else, and instead received this reply, which didn't answer his question. Regardless of whether one may think a guru's power of insight in replying to letters is supposed to be perfect or not, these three volumes are a superb introduction to Sri Aurobindo's philosophy and teachings, and include a great deal of important and very intriguing esoteric information, including the essay on the Intermediate Zone. Some have said that you need to read Sri Aurobindo's heavy books like Life Divine and Savitri to understand him, and that reading the Letters is, in a sense, well, if not "cheating" then...well, you get the idea. But I am of the school of thought that says you can read whatever you want at whatever level of difficulty you want, because it is whatever truth is transmitted to you that is important, rather than the letter of the law. And regardless of the status of the Letters I got heaps out of them - they were my bible for many years.