Hegel's Phenomenology of Mind ('Phenomenologie des Geistes': 1807). I have even uploaded a copy.
1. Because of its influence on later thinkers, such as Marx.
2. Because there was a great deal of interest in 'New Thought' books on the forum a few months ago. There were a lot of requests for Napoleon Hill's works. These are all 'think yourself rich' books and are generally based on a fundamental misunderstanding of Hegel. Read the real thing instead. You won't get rich, but you will think.