You are going to find much in way of Gandhi's writing and his thinking.  It's a bit difficult to narrow down his authorship because so much of his thought is present in writings, diary entries, essays, speeches, and other items.  There are "books," but the thought that Gandhi featured was prevalent in so much more than one genre of writing.  Like so many thinkers of the time period, he did not feel himself limited to one domain of written expression.  The link below is a fairly comprehensive list of his writings.  I would examine this extensively.  Of the works featured, there are some very interesting ones in terms of what they reveal about Gandhi's philosophy and value system.  "Hind Swaraj; or, Indian Home Rule" was one of the first articulations where he was on his way to South Africa as a lawyer and is a reflection about the nature of truth that he features in his own mind and in his own analysis.  It is interesting to see this as a potential "starting point," in terms of where "it all began."  Another interesting title on this list would be The Story of My Experiments with Truth. It is widely considered to be known as his most known book because it helped bring to the world stage the idea of Gandhi as both a freedom fighter, but also as one with a moral grounding, as well.  At the time of writing, Gandhi's campaign in South Africa had already been known and he was establishing himself as a figure in the Indian independence movement.  This book is powerful in how it provides a moral, ethical, intellectual, and spiritual grounding or justification to the political framework he was advocating in India.