I had to think deeply about this and particularly about a book changing the way I think about things and how often I think back to things in the book. For me it has to be Drawing On the Right Side of the Brain. It made me realize that I am more creatively skilled than I thought I was — a realization which has enhanced my life greatly. To some lesser degree The Artist's Way has influenced me as well.
Perhaps the most nuts-and-bolts useful of books for me has been John Saxon's Algebra 1 and Algebra 1 1/2 textbooks. After coasting through high-school algebra without gaining any real mastery, in my early 20s I sat down with these two masterworks of clarity and re-taught myself algebra in the space of a couple full weekends. I use this (re-)learning every day of my life.
A related book, and the basis for much of whats in Drawing on the Right Side of the Brain is The Natural Way to Draw, by Kimon Nicolaïdes. It is a text book and has structured lessons in it, so if you need formal structure its helpful. Following the lessons is not required to gain a lot from this book.
Perhaps the most nuts-and-bolts useful of books for me has been John Saxon's Algebra 1 and Algebra 1 1/2 textbooks. After coasting through high-school algebra without gaining any real mastery, in my early 20s I sat down with these two masterworks of clarity and re-taught myself algebra in the space of a couple full weekends. I use this (re-)learning every day of my life.