CC wrote 12 books in this order
The Teachings of Don Juan: A Yaqui Way of Knowledge
A Separate Reality: Further Conversations with Don Juan
Journey to Ixtlan: The Lessons of Don Juan
Tales of Power
The Second Ring of Power
The Eagle's Gift
The Fire From Within
The Power of Silence: Further Lessons of Don Juan
The Art of Dreaming
Magical Passes: The Practical Wisdom of the Shamans of Ancient Mexico
The Wheel of Time: Shamans of Ancient Mexico
The Active Side of Infinity
First 4 or 5 are more-less just an introduction and don't hold too much of power cause Castandeda only describes his experiences with don Juan. Books starting from Eagle's gift onward are what really holds the value here. The thing is, you probably want to start from the beginning to get a good base of terms used later but it's not mandatory.
If you wanna know more about the dreaming itself, when finishing the Art of Dreaming you can jump on Florinda Donner-Grau - Witch's dream. Florinda was don Juan's apprentice just as Castaneda was.
Be careful though, dreaming is not your very well know backyard. If you draw too much attention to yourself, the difference between this and the dream world will start to disappear and your awareness will be drawn back and forth like yoyo by these... well... entities