First one should read Works of the Mother, especially Volumes 1, 2 and 3. One may like to read all the other volumes in which case one should just go ahead reading them.
Alongside one can read some compilations such as Bases of Yoga, Elements of Yoga, Lights on Yoga, The Sunlit Path.
The first book of Sri Aurobindo that one should read is The Mother with Letters on the Mother, followed by Letters on Yoga and Sri Aurobindo on Himself. Then one can enter The Life Divine and Savitri.
This however is only in terms of apparent ease and level of difficulty. However different people connect differently for a number of reasons. Some start with Savitri, The Synthesis of Yoga, Prayers and Meditations and The Life Divine. Others begin with small compilations such as you may find on auromaa.
One has to see how one feels connected to the book as these are living words and draw us depending upon our inner needs.
Affectionately
Alok da
Source:
श्री अरविंद सनातन शतदल (TH 348)
YT Link: https://youtu.be/Rl7KHB3qV9Y