I buy a lot of books (a colleague in the US has even claimed a correlation between the arrival of Amazon boxes at my cube and Amazon's share price). I don't necessarily read them all cover-to-cover. Someone once said to me that a single idea on a single page of a book and the book can pay for itself.
Here are three books that contained an idea that really made me change my mind about something. They come from complete different subject areas but they each gave me a real "wow" moment.
The Wealthy Barber by David Chilton — when I asked an accountant for financial advice back in 1997, he said "just read this book". The book has some great ideas but the real "wow" moment for me was the stuff about life insurance.
Geometrical Vectors by Gabriel Weinreich — when I was teaching myself differential geometry to understand General Relativity, I struggled to get an intuitive grasp of the distinction between vectors, one-forms, cross-products, etc. This book completely changed the way I think about vectors and vector calculus (I wish I'd had it in 2nd year mathematics).
The Epistles of John by Raymond Brown — I devoured the copy of this in the University Library when I was an undergraduate student. Not only did it change my mind about dealing with false Christian teaching but instilled in me a real fascination for reconstructing the context of the New Testament epistles.
There are other books that have had similar impact too. I'll post about them another time.