Not nearly as rarefied as many of the books cited here, for me it was John Saxon's excellent Algebra 1/2, Algebra 1, and Algebra 2. I didn't get a good enough g

Not nearly as rarefied as many of the books cited here, for me it was John Saxon's excellent Algebra 1/2, Algebra 1, and Algebra 2. I didn't get a good enough grasp on basic Algebra in high-school. When I was in my early–mid 20s, a friend gave me these three algebra textbooks. In a marathon session lasting about two weeks, I went through all three books from end-to-end and really learned algebra. For whatever reasons, the Saxon books worked really well for me — better than any other learning I have ever gotten from a textbook. Although I was very motivated, I attribute a lot of my success learning algebra well to those three books. I still own them.