For personal development I would recommend any book which helps us to understand ourselves better. Once we know who we are it becomes easy to choose a path to self development.
1) The Alchemist - by Paulo Coelho.
This book helps us understand about inner voice. We have all the necessary senses which can help us to listen to this inner voice. But after some time we turn deaf towards this voice or we stop listening.
This book helps us to identify this voice. With the help of a beautiful story it shows us the path to achieve what we are capable of.
2) Positive Intelligence - Sirzad Chamine
Great book about our mind. It is not about positive thinking. I use the concepts mentioned in this book while answering many questions about human behaviour.
It helps us to identify two parts of our brain. One which is judgemental, volatile, reflexive, unmerciful, etc. This book helps us realize the importance of this part of brain which everyone relates to. This is only part which is highly developed. It is the cause of anger, hatred, and all the various negative emotions.
This book also lets us meet another part of the brain which is forgiving, merciful, deep thinker and all the positive emotions which we sometimes feel are product of this part.
The book has practical advices to uncover hidden potential of the other part of our mind.
3) Think and Grow Rich - by Napolean Hill.
Only comment on this book I can come up with is that it is an ageless classic.
The secret to greatness is effectively brought out in this book. Only that it is like hidden treasure. You have to read and read it again to uncover the secret.
4) The Goal - by Dr. Eliyahu M. Goldratt
This is a book preferred by management students. A book which highlights about productivity. The way the concepts are presented using a great story is commendable.
If this book is read to absorb knowledge then apart from productivity there are many life lessons which can be extracted out of this gem.
5) The works of Swami Vivekanand
I would have mentioned Bhagvat Geeta. It is pure philosophy and a bit difficult to relate to.
If we read works of Swami Vivekanand then most of the philosophies mentioned in Geeta is explained in simple way. The way we can understand and visualize. The PDF is easily available on internet.
And lastly the greatest asset which we can use to learn is our brain. The deeper we try to observe everything and learn, the more we will uncover and know. No book can replace the power of our mind.
------- Last advice.
Keep looking outside but never forget to explore the inside. Happy exploring.
P. S. - What are your favorites. Please share in comments.