The real world is continuous, but computers are discrete. The magic of computational science lies in discretization—turning a smooth curve into a set of points (nodes). Strang excels at explaining this process. He simplifies the complex notation often found in numerical analysis texts, allowing the reader to see the underlying patterns.
While not "free," older editions of Computational Science and Engineering are extremely cheap on AbeBooks, eBay, or ThriftBooks. You can often find a used international edition for under $20. The real world is continuous, but computers are discrete
Moving from continuous physics to discrete computation. The real world is continuous